Ultrathin Co3O4 Nanosheets with Edge-Enriched {111} Plane as

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Ultrathin Co3O4 Nanosheets with Edge-Enriched {111} Plane as Efficient Catalysts for Lithium-Oxygen Batteries Yue Zheng, Rui Gao, Lirong Zheng, Limei Sun, Zhongbo Hu, and Xiangfeng Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b05182 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Catalysis

Ultrathin Co3O4 Nanosheets with Edge-Enriched {111} Plane as Efficient Catalysts for Lithium-Oxygen Batteries Yue Zhenga‖, Rui Gaoa‖, Lirong Zhengb, Limei Sunc, Zhongbo Hu, Xiangfeng Liua*

a College

of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

b Beijing

Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

c Department

of Nuclear Physics, China Institute of Atomic Energy, Beijing 102413, China

‖ These two authors contributed equally to this work.

*Corresponding Author: [email protected]

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Abstract: Li-O2 batteries have aroused great interest as promising chemical power sources due to the high energy density. But how to enhance the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on cathodes is still a big challenge. Herein, we design and prepare Co3O4 single-crystal nanosheets with edge-enriched {111} facets via a simple pyrolysis of α-Co(OH)2, which show a high activity as cathode catalysts for Li-O2. The catalytic activity and stability have been significantly improved in compared with Co3O4 poor-edge {111} facts. This enhancement can be largely attributed to the edge-enriched {111} facets. Co3O4 nanosheets with edge-enriched {111} facets expose more metal atoms to boost the catalytic activity. Moreover, the edge-enriched {111} facets contain a high density of atomic steps and kink atoms which supply a wide space for breaking chemical bonds and increasing the reaction activity. This study also presents some insights into designing high performance catalysts for metal air batteries through a strategy of edgeenriched facets engineering.

Keywords: Li-O2 battery; edge-enriched facets engineering; catalyst; Co3O4; nanosheets

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INTRODUCTION Nowadays, rechargeable metal-air batteries especially Li-O2 batteries have attracted much attention because of their superior high energy storage density.1-8 However, the slow kinetics of oxygen evolution reaction (OER) and oxygen reduction reactions (ORR) for the decomposition and formation of Li2O2 result in the low reaction efficiency and the large polarization potential between charging and discharging process.9-12 Moreover, the poor cycle stability also restricts its practical application in electric vehicles and other aspects.13 Up to now, some strategies have been explored to develop various cathode catalysts to solve the cathode problem of Li-O2 battery, such as nanocrystalline electrode, defects, facet engineering, heterostructure and so on.14,15 For example, Cao et al.16 used porous MnCo2O4/Mo nanosheets grown in nickel foam as cathode of LiO2 battery. The results show that the nanocrystalline electrode has good cycling, rate performance and high energy efficiency (more than 85%) as carbon-free electrode. By constructing an in-situ growth of α-MnO2/RuO2 composite on graphene nanosheets, Cai et al find that the synergistic catalytic effect between the α-MnO2/RuO2 which enhances the OER performance and obtains a lower over potential.17 Zhang et al. find that RuO2 nanosheets loaded on La0.6Sr0.4Co0.8Mn0.2O3 nanofibers can enhance both ORR and OER performance of battery and even can effectively decompose side products.18 Among these methods, crystal-facet effect is especially significant, because it is always an effective strategy to construct active plane to enhance the performance in the field of catalysis.19,20 It has been found that the active crystal planes with unstable 4


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hanging bonds and high density atomic steps at the edge can rapidly transfer ions between the surface and the structure. Liu et al.21 reported that the synthesized Cu2O concave octahedron with {332} high-index facets showed a significant improvement in the catalytic oxidation of CO in compared with cubic Cu2O crystallites. Xie et al22 find that the crystal plane of [110] of Co3O4 nanorod benefit the catalytic reaction of lowtemperature oxidation of CO, mainly owing to the more Co3+ sites. Some other studies have proved that the designed crystal planes, especially the doped plane, the defective plane and the plane with rich-edge effect have more adsorption energy and more adsorption sites, which shows the high potential in the electro-catalytic activity.23-28 Cobalt tetroxide, which is composed of mixed valence state with tunable Co3+ and Co2+, has unique electronic catalytic properties compared with other transition metal oxide based cathode catalysts.29-31 The crystal plane effect of Co3O4 has always been concerned in the field of energy storage and conversion, and other catalytic fields.32 Hu et al.33 found that Co3O4 exposing the crystal plane of {112} exhibited a better performance than other planes because {112} plane expose more atomic steps and Co active sites. Gao et al34 and Su et al35 confirmed that as for Li-O2 battery catalyst, [111] plane of Co3O4 shows a better electrochemical performance owing to its high surface atomic density. In addition, the surface atom state and atom edges also largely affect the electrocatalytic performance. Co3O4 with high-index plane and defective plane may show excellent activity for the rich atomic boundary and more active sites.

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However, it's still a thorny challenge to avoid too rapid surface energy reduction to generate exposed crystal planes due to the fast-growing of edge-enriched facets with 5


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high surface energy.38-41 Herein, we construct an edge-enriched Co3O4 exposed {111} plane (Co3O4-H) by calcining anion intercalated α-Co(OH)2. Compared with the as prepared Co3O4 nanosheets with {111} plane, Co3O4-H shows not only high active crystal plane of {111} but exhibits edge enriched state. Edge-enriched structure was clearly observed by HAADF-STEM and the formation mechanism of the rich-edge was studied by ex-situ XRD and TGA. Compared with Co3O4 exposed {111} fact with poor edges, Co3O4-H exhibits a high catalytic activity and stability mainly because of more metal atoms exposed, which promotes the catalytic activity. Furthermore, the edge-enriched {111} facets contain a high density of atomic steps and kink atoms which further provides a wide place for breaking chemical bonds and increasing activity. The proposed edgeenriched facts engineering strategy in this study offers some new insights into the design of efficient catalysts for metal air batteries and other catalysis fields. EXPERIMENTAL SECTION Synthesis of Materials α-Co(OH)2 was obtained as reported before.42, 43 In this experiment, 0.3418 g NaCl, 0.5844 g CoCl2·6H2O and 1.68 g hexamethylenetetramine (HMT) were dissolved in 200 ml DI water and ethanol (9:1). Then the mixture was heated to 90 °C by magnetic stirring and reacted for 1 h to obtain green suspension. It was then centrifugated and washed with water several times. Finally, the precursor α-Co(OH)2 was dried at 80 °C for 8 h. To obtain the Co3O4 crystal nanosheets with edge-enriched {111} facts, the resulting product was calcined in the air at the temperature of 450 °C for 2 h. Co3O4 6


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with high facets was labeled as Co3O4-H. Co3O4 with poor-edge {111} facets was obtained through a previous method. 44 0.291 g Co(NO3)2·6H2O was dissolved in 20 ml DI water to form a red solution, and then 2 ml octadecenylamine and 10 ml ethanol were added under magnetic stirring for 1h to form a green solution. After that, the solution was transferred to an 40 ml autoclave for at 180 °C for 12 h to get β-Co(OH)2 nanoparticles. At last, β-Co(OH)2 nanoparticles were calcined in air at 450 °C for 2 h to obtain Co3O4 nanosheets. To avoid confusion Co3O4 with poor-edge {111} facts was labeled as Co3O4.

Structure characterization of the samples X-ray diffraction (XRD) was tested on a Rigaku Smartlab powder diffractometer with Cu Kα radiation (λ = 0.154 nm, 9 kW), 2θ ranging from 10 to 80°. X-ray photoelectron spectroscopy (XPS) was used to display the binding energy of electrons and to provide information on functional groups and valence states, collected on Thermo Escalab 250Xi with a monochromatic X-ray source (Al Kα hυ = 1486.6 eV). Besides, all spectra were normalized based on 284.8 eV of carbon peak. Thermogravimetric analysis (TGA) was used to analyze the forming process changes of materials through Meittler SF/1382. The morphology of the catalysts was observed by a scanning electron microscope (SEM, Hitachi SU-8010) and transmission electron microscope (TEM) using on a FEI Tecnai G2 F20 S-TWIN (200 kV). The test parameters of FE-SEM was 5kV, 10uA. The high resolution spherical transmission electron microscope (HRSTEM) is eliminated by spherical aberration, and the atomic

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structure image with higher resolution is obtained. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images were performed on Titan themis

with acceleration voltage of 300 kV. The surface area of the powder

was calculated by the Brunauer−Emmett−Teller (BET) method on a Micromeritics ASAP-2460. Differential electrochemical mass spectrometry (DEMS) were carried out on i-DEMS 100 (Linglu) with EI-70ev ion source and SEM-100v Detector. Fabrication of oxygen cathode and electrochemical measurements The electrochemical performance of Li-O2 cells with different catalysts was measured with a special designed 2032 coin cell. The whole battery consists of porous air electrodes, lithium foil anodes, glass filter separator and 1.0 M LITFSI/TEGDME electrolyte. The cathode material was mixed with NMP in a certain proportion, after that the above catalyst slurry, binder PVDF and super P carbon were prepared at 4:2:4 ratio. The porous air electrodes which coated with the mixture were first dried for 3 h at 80 °C, then dried at 120 °C for 12 h. The coin cell were assembled in a glove box filled with argon (Eteluxx LAB2000). Besides, the contents of water and oxygen should be less than 0.1 ppm. Battery test system was tested by the LAND CT2001A (5V, 5mA), the galvanostatic charge-discharge process of the battery was carried out at room temperature and in the voltage range of 2.0 to 4.5 V. The cut-off cycle test was carried out at the current density of 100 mA/g. The capacity limit was 500 mAh/g. All the cells needed a series of tests under a dry pure O2 atmosphere with 1.0 atm. The capacity and current density were calculated on the basis of the mass of super P carbon and catalyst loaded on the cathode. 8


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RESULT AND DISCUSSION

Figure 1. Illustration of the preparation of ultrathin Co3O4 nanosheets with edge-enriched {111} plane.

Figure 1 shows a scheme of the complete preparation process of hexagonal Co3O4 nanosheets with edge-enriched facets. In a typical process, Cl- interted α-Co(OH)2 was first obtained with a shape of hexagonal nanosheets through the method of precipitation. Ultrathin Co3O4 nanosheets with edge-enriched {111} plane (marked as Co3O4-H) was finnally obtained by calcinating the α-Co(OH)2 precursor. In order to compare the electrochemical performance of this Co3O4 and other Co3O4 nanosheets, another pooredge plane Co3O4 nanosheet, which also exposed {111} plane, was prepared (marked as Co3O4) according to previous research44. Figure 1B also provide the formation mechanism of edge-enriched structure, which is mainly divided into three steps and the details of the formation will be discussed later. 9


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Figure 2. FESEM and HRTEM images of Co3O4-H and Co3O4. a, b, c,) The FESEM images of αCo(OH)2, Co3O4-H and Co3O4. d, e) The HRTEM image of Co3O4-H and Co3O4. The insert figures of d) show the corresponding SAED pattern. The insert figures of d, e) show the lattice spacings of them. f, g, ) The HAADF-STEM images of selected edges

Figure 2a-c show the FE-SEM images of α-Co(OH)2 precursor, edge-enriched Co3O4 nanosheets and poor-edge Co3O4 nanosheets. Figure 2d, e correspond to the HRTEM pattterns of Co3O4-H and Co3O4. It is clear that the hexagonal structure of the 10


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material before and after calcination does not change much. In addition, many pore structures were etched after pyrolysis. The nanosheets are about several micronmeters in 2D and a few nanometers thickness. From the Selected Area Electron Diffraction (SAED) and HRTEM images of Co3O4 -H and Co3O4 nanosheets, it can be shown that Co3O4 nanosheets were lying on different crystal planes and taken along the [111] zone axis direction. The insert figure shows the lattice of Co3O4. The interplanar spacing of Co3O4 -H reflect the index of [440] (1.43 Å) while Co3O4 reflects the index of [220] which is 2.86 Å. In order to more clearly observe and compare the inner and edge differences of Co3O4, the HAADF-STEM patterns are taken to show the detailed structure of two kinds of Co3O4. By highlighting the edge of Co3O4 nanosheets on the TEM images (Figure 2f and g), we find that Co3O4-H shows more edges than that of Co3O4. Figure S1 presents the whole morphology in a low magnification and clearly shows the rich-edge structure. That’s really different from the smooth edges of Co3O4. Compared with Co3O4, Co3O4-H which exposes a higher-index of crystal plane (440) has a smaller interplanar spacing, a large number of step surfaces, and more Co2+ on its surface. This may benefit to provide more active site that attracts more O2. Moreover, Figure 3a and b represent the HAADF-STEM patterns of Co3O4-H and Co3O4, respectively. It can be seen that Co3O4 with more edges facets has large six-member ring (blue) and slightly less obvious small six-member ring (green), which represent Co3+ and Co2+, respectively. However, Co3O4 with poor edges facets has only a large six-member ring structure, while small six-member ring structure can hardly be seen, because the crystal band axis of Co3O4 has a certain deviation and therefore less Co2+ 11


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is exposed. We further use the crystal cell structure image to illustrate the difference between the two kinds of Co3O4 with different crystal planes on their surfaces as shown in Figure 3c and d. In these figures, pink plane reflects the plane of (220), while the blue plane reflects the plane of (440). As mentioned before (Figure 2d and e), the spacing of Co3O4 plane with [440] is exactly half of the plane with [220]. What's more interesting to us is that the plane of [440] contains more Co2+, as seen in the locally enlarged front and side images of Figure 3e and f, which can explain the existence of the zigzag atomic ladder in the nanoscale.44 Figure 3g and 3h, and Figure S2 clearly show this result and reflect the unsaturated coordination on the edges. In compared to well-defined Co3O4-H, Co3O4 only has the mixed layer. The yellow arrows reflect the plane of [440]. In Figure S2a of Co3O4-H, the cobalt can be clearly identified and the red arrows of the selected area reflects the exposing of Co2+. However, as for Co3O4 in Figure 3h, the plane of [440] are not clearly be identified and almost no obvious Co2+ can be found. This structure also makes the surface of Co3O4-H expose more Co2+, which is beneficial to the reaction of OER and ORR. Considering the potential effect of the porosity and particle size on the catalytic performance, BET was carried out to show the difference in the surface areas. The results show that Co3O4 with a small size exhibit a larger surface area of 24.61 m2/g while Co3O4-H has a much smaller surface area than Co3O4. (Figure S3)

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Figure 3. Illustration of the difference of different index facets between Co3O4-H and Co3O4. a, b) HAADF-STEM atomic scale image of lattice fringe and cell structure of Co3O4-H and Co3O4. c, d, e, f) Cell structure diagram with different angles. g, h) The atom state of edge of Co3O4-H and Co3O4

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Figure 4. The XRD patterns of a) the formation of Co3O4 observed by changing the calcination temperature, b) The TGA pattern of Co3O4 formed in stages. c) XRD patterns of precursor, Co3O4 and Co3O4-H. d) The XAFS patterns of Co3O4-H and Co3O4.

The formation process of edge-enriched structure will be discussed based on the analysis of XRD and TGA. Figure 4a shows the XRD patterns of the formation of Co3O4 nanosheets with temperature changing. It’s abviouly seen that the structure of the precursor was observed by changing the calcination temperature. The experimental results show that the diffraction peak at 11° disappears and the diffraction peak at 38° appears. This process can also be confirmed by TGA patterns (Figure 4b). The main curve can be separated into three main process as illustrated in Figure 1B. In the first process, the layered structure can still be maintained and some inserted water molecules run out. At this point, the position of the main peak of the (003) plane of the precursor did not change, but the intensity of the other peaks began to decrease. When temperature rise 180°C (the second process), Co(OH)2 begins to decompose. OH- and Cl- transferred into gas and escaped. Some phase of Co3O4 formed. At the same time, the characteristic peaks of the precursor begin to disappear, and the whole peak position changes to the left. After 250 °C, all the Co-ion migrate from lamellar to interlamination. It can be seen that the peak shape of Co3O4 begins to be obvious and by comparing with the standard PDF card, the final calcination is the formation of pure Co3O4. However, because of the long migration distance of Co ions to the interlamination and the spill of Cl- and OH- as gas, discontinuous macropore appeared and more Co atoms are exposed to the surface of Co3O4, forming the edge-enriched crystal surface. Figure 4c provide the XRD patterns of Co3O4 nanosheets which can be indexed in the standar PDF No. 14


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74-2120. Moreover, the structure of α-Co(OH)2 in this paper reflects a typical characteristic of lamellar hydroxides. The main peaks at about 11° (2-theta) indicating the (003) d-space of this obtained α-Co(OH)2 are 7.9 nm. After calcination, the thickness of Co3O4 nanosheets was obtaind. It should be noticed that, as for XRD, the orientation of single exposed crystal surface will cause the change of relative peak strength.46 However, the powder XRD test we've taken showed that the sample particles were stacked at random on the sample holder, so the diffraction of each orientation would occur. The results of the powder diffraction of the two kinds of Co3O4 materials are not obviously different in the end. Figure S4 provides the XRD patterns after tiled Co3O4 nanosheets on sample holder. The strong peaks of (111) clearly reflect Co3O4-H exposes the plane of {111}. Though the phase of catalyst was preliminarily analyzed by XRD, the highly dispersed Co3O4 was characterized by XAFS because of the limitation of XRD test to some extent. Figure 4d provides the XAFS patterns of Co3O4H, Co3O4 and Figure S5 shows the EXAFS date of Co3O4-H, Co3O4. The XAFS and EXAFS mainly reflects the information of the cluster structure in a small area, all the peaks show a similar shape, indicating that after calcination, the main chemical environment of Co has not been largely changed.

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Figure 5.a) Co 2p of XPS spectra of Co3O4-H and Co3O4. b) O 1s for Co3O4-H and Co3O4.

X-ray photoelectron spectroscopy (XPS) can reflect the surface state of Co3O4. As shown by the X-ray photoelectron spectroscopy (XPS) in Figures 5a, the two major peaks are at 780 and 795 eV, corresponding to Co 2p3/2 and Co 2p1/2, respectively. As for Co3O4-H, the peak position shifts to the direction of low binding energy compared with Co3O4 with more Co2+/Co3+, indicating there are more Co2+ formed. Moreover, Figure 4b shows three oxygen peaks of O 1s region of Co3O4 nanosheets, corresponding to lattice oxygen (529.9 eV), surface oxygen vacancy (531 eV) and surface chemisorption or dissociation oxygen (532.8 eV), respectively. From the macro perspectives, the Co/O ratios of the two materials are 0.429 (Co3O4-H) and 0.426 (Co3O4), respectively, which means, O-rich or O-deficiency zones are not the main factor. However, by comparing the upper and lower samples in detail, it’s obvious that the XPS strength of surface oxygen vacancy increases for Co3O4-H, which indicates the introduction of some oxygen vacancy sites, mainly form the defect of edges.47, 48 Apparently, more Co3+ exposed by Co3O4-H could be reduced to Co2+ with more oxygen vacancies formed on the edges, as we observed on STEM images. Above all 16


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the result of XAFS, XPS, BET and STEM we can conclud that the two Co3O4 preared using different synthesis methods share the same crystal direction and a similar chemical environment. The difference of the performance is mainly caused by the edge structure. 49

Figure 6.a) The first capacity cut-off discharge/charge curve. b) Comparison of cycling performance of the two kinds of Co3O4. c) The cycle performance and discharge stability of Co3O4-H and Co3O4 based battery at the current density of 200 mA/g.d) The first capacity cut-off discharge/charge curve of Co3O4-H with LiI measured at the current density of 200 mA/g.

From initial galvanostatic ORR–OER profiles in Figure 6a, Co3O4-H and Co3O4 show similar overpotential as the cathode catalyst when the lithium-air batteries are charged and discharged deeply at the current density of 200 mA/g. The first discharge/charge capacities of the cathode catalyzed by Co3O4-H crystal nanosheets are 5220 and 5159 mAh/g, respectively. But the initial discharge/charge capacity of Co3O4based cell are 3723 and 3816 mAh/g correspondingly. This can reflect Co3O4-H with a smaller surface area has more active sites and more ability to store Li2O2 because of the 17


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riched-edges effects. Figure 5b shows the cycling performance of Co3O4-H and Co3O4. By analyzing the profiles of Figure 5b, it can be clearly seen that the discharge capacities of Co3O4-H and Co3O4 after 15 cycles at a rate of 100 mA/g are 3653 and 2886 mAh/g. In general, the discharge capacity of Co3O4-H is always higher than that of Co3O4 during the cycles, indicating that Co3O4 with edge-enriched {111} facets possess the higher charge/discharge capacity. It should be noticed that the overpotential is always increasing as the cell works on. This phenomenone is mainly coused by the decomposion of electrolyte at a high voltage and the fade of Li metal. Almost none of current electrolyte can reach a coulombic efficiency of 100%.50 On one hand, the higher oxidability of the intermediate products (O2-, O22- etc.) and strong reducing property of metal Li can react with electrolyte.51 On the other hand, electrolyte can be oxidized at high potential. So, it is difficult to conclude much only from the cycle life. Figure S6 shows the quantitative measurements of oxygen evolution by using differential electrochemical mass spectrometry (DEMS). The result shows that in this system, at a lower overpotential, the recharge process is mainly caused by the decomposion of Li2O2, while at the higher voltage (above 4.0V), CO2 appears because of the side reaction of electrolyte. This result also confirmed that in this Li-O2 battery, electrolyte oxidation is not the dominant reaction when the capacity is limited. But with the increase of the capacity especially at the end of recharge, electrolyte decomposes seriously. Figure S7 provides XRD patterns and FTIR patterns after 5 limited-cycles. No obvious side product can be indexed which clearly shows a better ORR and OER performance of Co3O4-H. 18


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Figure 7. The capacity cut-off discharge/charge curve of the cathode catalyzed by Co3O4-H and Co3O4, measured at the current density of 200 mAh/g when the capacity is limited to 500 mAh/g.

In order to decrease or even avoid the influence of capacity loss caused by deep discharge, we conducted galvanostatic discharge–charge cycling experiments to test the electrochemical performance of Co3O4-H and Co3O4 nanosheets. Figure 7 shows the voltage-capacity profiles of Co3O4-H and Co3O4 nanosheets limitd to capacity of 500 mAh/g. Under limited capacity conditions, the charge potential of Co3O4-H nanosheets at the first cycle is less than 3.7 V, and the discharge potential at the first cycle is 2.6 V. The initial charge-discharge potential of Co3O4 nanosheets are at about 4.3 V and 2.6 V, respectively. It means that Co3O4-H nanosheets with edge-enriched {111} facets and high densities of atom steps and more surface active sites has a stronger absorption absorption and lower overpotential than Co3O4 with poor-edge {111} Co3O4. However, because of accumulation of a large number of discharge products on the surface of the cathode it requires higher over-potential with cycles. For example, the charge overpotential of Co3O4-H is close to that of Co3O4 at the 70th cycle, whereas Co3O4-H is in general more excellent in promoting the fast conversion of oxygen. There is no doubt that the number of cycles in Co3O4-H can reach more than 100 cycles, while the 19


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discharge voltage of Co3O4 begins to significantly decline at 70th cycle and can’t even reach the cut-off capacity. Overall, the cycling stability of cathode catalyst based on Co3O4-H crystal nanosheets with edge-enriched {111} planes was significantly enhanced in compared to Co3O4 crystal nanosheets with poor-edge {111} plane, probably due to more active sites on the surface and larger specific surface area. Figure 8 a-f shows the morphologies of Co3O4-H and Co3O4 as cathode catalyst during charging and discharging. It can be seen that Co3O4 with edge-enriched {111} planes retains the hexagonal porous structure more completely (Figure 8a) and a layer of Li2O2 was formed on the surface of Co3O4 because of rich edge on the nanosheets and uniformly adhered when the discharge capacity reached 500 mAh/g (Figure 7b,c). As the discharge progressed, the surface attachments inclined to become toric structure, indicating that discharge products-Li2O2 were deposited more and more, then the surface of layered structure disappeared after charging which meaned Li2O2 decomposition and the catalyst surface restitution at the same time. From the SEM images, we noticed that at the end of discharge, Li2O2 grows in toroidal structures. Accoring to previous reports, the morphology of Li2O2 can be affected by electrolyte solvents or catalysts.52, 53 In contrast, when the Co3O4 catalyst with poor-edge facets was discharged on the electrode, the sphere-like structure similar to the cluster was formed on the surface and then there was also toric structure formed after full discharge (Figure 8d-f). It could be clearly found that in this cathode, Li2O2 did not grow along the crystal plane of {111} of Co3O4. The above result shows that at a low capacity, the surface strcture of the catalysts has a large effect on the formation and morphology of 20


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ACS Catalysis

Li2O2. However, with the increase of capacity, more Li2O2 formed and the surface structure has litttle effect. Trace amount of water from gas or electrolyte may lead to the formation of toroidal Li2O2 when discharge in deptch. But the final mophology of Li2O2 is similar. Therfore, some residues are not the main factor in this study because the same electrolyte is used in the battery tests and the surface structure of the catalysts has an important effect at the begining of Li2O2 formation. In a nutshell, the discharge products (Li2O2) of Co3O4 with edge-enriched {111} planes used as a cathode catalyst are flimsy and uniform, result in accelerating to decompose during charging, thereby promoting the conductivity and catalytic performance of the electrode. This also indicated that Li2O2 always depososed on the site of edge of Co3O4 and Co3O4 exposing edge-enriched {111} plane can exhibit a better performances.

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Figure 8.a, b, c) SEM patterns of charge/discharge process of cathode catalyst based on Co3O4 -H crystal nanosheets. a, d) The SEM images of blank Co3O4-H and Co3O4. b, e) discharge capacities of 500 mAh/g. c, f) full discharge. The insert figures of a) and d) show the cathode catalyst after charging. 1, 2) corresponding process diagram.

The decomposition of product Li2O2 produced in the process of battery charging and discharging is of great significance to evaluate the performance of air batteries.50 Therefore, we studied the potential reactivity between Li2O2 and Co3O4-H catalyst, especially on OER process. In order to further discuss the OER between Li2O2 and Co3O4-H, we also directly added Li2O2 to Co3O4-H to simulate the recharge process. Figure S8a and b are the recharge profile and LSV curve of Li2O2-contained battery. From the profile we found that under the catalysis of Co3O4-H, Li2O2 can obtain an oxidation peak of 4.3 V. XRD in Figure S8c shows the comparation of Li2O2 -contained 22


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ACS Catalysis

cathode before and after recharge. As shown in the XRD pattern, Li2O2 can be clearly identified in pristine cathode, while after recharge, the diffraction peak disappeared totally. This reflects the good OER activity between Li2O2 and Co3O4-H catalyst. The result of EIS also agreed with the result. After recharge process, Li2O2 decomposes and the impedance decreases. (Figure S8d)

Figure 9. SEM patterns of charge/discharge process of cathode catalyst based on Co3O4-H with LiI crystal nanosheets at different stages. XRD pattern of the corresponding charge-discharge reaction processes.

In order to reduce the charge-discharge voltage difference to further improve the battery performance, we added 0.1M LiI as an additive to the Co3O4-H electrolyte on the basis of the original battery system. According to the report, the introduction of LiI can change the OER process of the battery to get better battery performance.54, 55 From Figure 6d, we can see that the discharge platform is still around 2.6V in the first discharge process, indicating that the reaction are Li and O2 to generate Li2O2 as before. However, the battery based on Co3O4-H added with LiI shows a lower charging platform about 3.6V in the first charging process, lower than the primary Co3O4-H. 23


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Figure 9 indicates that the surface of Co3O4-H with LiI is smooth before discharge, showing a porous hexagonal structure. After recharging at 500 mAh/g and 1000 mAh/g, the surface of the electrode formed patchy structure attached to cotton floc- massive oxidation products Li2O2 and LiOH. And then patchy structure tends to be smooth with charging, showing that the decomposition of the hydroxides and peroxides of Li. After complete charge and discharge, the surface of the electrode is smooth. In summary, LiI acts as a redox mediator and cooperates with the catalyst Co3O4-H during the charging reaction, which facilitates the decomposition of Li2O2 at a low potential. CONCLUSION Co3O4 nanosheets with edge-enriched {111} plane generate more Co2+ that can adsorb O2, and expose more active sites and atom-step densities for ORR and OER reaction. These make Co3O4 with edge-enriched {111} plane have more significant electrocatalytic properties as a cathode catalyst for Li-O2 battery. The initial capacity and the cycling stability of the battery based on Co3O4-H with edge-enriched {111} are improved in compared to Co3O4 with poor-edge {111} index planes, and the overpotential is also reduced. The proposed edge-enriched facets engineering strategy to enhance the catalytic activity may also be applied to tune some other catalysts. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. TEM images of Co3O4 and Co3O4-H, the atom state of edge of Co3O4-H and Co3O4, the BET profiles of Co3O4 and Co3O4-H, XRD patterns of powder Co3O4 samples and 24


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ACS Catalysis

samples on orientation distribution, EXAFS profiles of Co3O4 and Co3O4-H in k-space, gas evolution rates (left) and galvanostatic charge curves of Co3O4-H based battery, XRD and FTIR patterns of cathodes after 5 limited-cycles, charging curves and LSV curve of electrodes containing Li2O2, XRD patterns and EIS of cathode containing Li2O2 before and after charging. AUTHOR INFORMATION *Corresponding Author: [email protected]. ACKNOWLEDGMENT This work was supported by Natural Science Foundation of Beijing (Grant No. 2182082), National Natural Science Foundation of China (Grant No. 11575192), the Scientific Instrument Developing Project (Grant No.ZDKYYQ20170001), the International

Partnership

Program

(Grant

No.

211211KYSB20170060

and

211211KYSB20180020) and “Hundred Talents Project” of the Chinese Academy of Sciences. We thank Dr. Zhang from ZKKF (Beijing) Science & Technology Co., Ltd for TEM and HAADF-STEM observations and Mr. Zhang from Linglu Instruments (Shanghai) Co.,Ltd. for DEMS testing.

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Ultrathin Co3O4 Nanosheets with Edge-Enriched {111} Plane as

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