Toward Highly Efficient Electrocatalyst for Li–O2 Batteries Using

Apr 12, 2017 - For the promotion of lithium–oxygen batteries available for practical applications, the development of advanced cathode catalysts wit...
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Toward Highly Efficient Electrocatalyst for Li−O2 Batteries Using Biphasic N‑Doping Cobalt@Graphene Multiple-Capsule Heterostructures Guoqiang Tan,†,# Lina Chong,†,# Rachid Amine,‡,§,# Jun Lu,*,† Cong Liu,† Yifei Yuan,†,∥ Jianguo Wen,⊥ Kun He,∥ Xuanxuan Bi,† Yuanyuan Guo,† Hsien-Hau Wang,‡ Reza Shahbazian-Yassar,∥ Said Al Hallaj,§ Dean J. Miller,⊥ Dijia Liu,*,† and Khalil Amine*,† †

Chemical Sciences and Engineering Division and ‡Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States § Department of Chemical Engineering and ∥Department of Mechanical and Industrial Engineering, The University of Illinois at Chicago, Chicago, Illinois 60607, United States ⊥ Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: For the promotion of lithium−oxygen batteries available for practical applications, the development of advanced cathode catalysts with lowcost, high activity, and stable structural properties is demanded. Such development is rooted on certain intelligent catalyst-electrode design that fundamentally facilitates electronic and ionic transport and improves oxygen diffusivity in a porous environment. Here we design a biphasic nitrogen-doped cobalt@graphene multiple-capsule heterostructure, combined with a flexible, stable porous electrode architecture, and apply it as promising cathodes for lithium−oxygen cells. The biphasic nitrogen-doping feature improves the electric conductivity and catalytic activity; the multiple-nanocapsule configuration makes high/uniform electroactive zones possible; furthermore, the colander-like porous electrode facilitates the oxygen diffusion, catalytic reaction, and stable deposition of discharge products. As a result, the electrode exhibits much improved electrocatalytic properties associated with unique morphologies of electrochemically grown lithium peroxides. KEYWORDS: MOF, cobalt nitride, N-doped graphene, cathode catalyst, lithium−oxygen battery

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sputtering can hardly distribute catalysts onto supports uniformly, thus resulting in a low utilization of cathodes.28,29 Recently reported freestanding metal oxides grown onto carbon fibers or nickel foams show the promising application in Li−O2 cells but they easily suffer from the low electric conductivity and fast structure degradation.29−31 Therefore, up to now very few cathode materials and their architectures have been developed toward an optimized combination of the abovementioned critical parameters, and the underlying mechanism is still in ambiguity. Considering the challenges for cathode catalysts, metal− organic framework (MOF) materials featuring simple synthesis and uniform structure offer a broad choice of catalyst materials for Li−O2 cells.32,33 However, further optimization of materials and structures should be undertaken to improve their electrochemical performance. Moreover, a fundamental study

ithium-oxygen (Li−O2) batteries is being extensively studied due to their highest theoretical energy density in current existing electrochemical energy storage systems.1−5 Considerable research effort has been dedicated to Li−O2 systems, including the mechanism understanding,6−12 materials development,13−21 and cathode architectural design.22−24 From the catalysis point of view, the catalysts indeed play a significant role for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as well. However, these catalysts mainly included noble metals and their oxides, introducing significant cost on the material fabrication side. In addition, most of catalysts have limited function only in ORR rather than in OER where the overpotential is typically much more severe. For example, compared to the pure carbon, no difference in catalytic activity for the OER is observed by using Au, Pt, or RuO2 catalysts.25−27 The architecture of catalyst−cathode is paramount important for Li−O2 cells, because the cathode should provide enough channels for the transport of Li+ and oxygen and guarantee enough space for the deposition of discharge products from electrochemical reactions. However, traditional methods such as the mechanical mixing and © XXXX American Chemical Society

Received: January 16, 2017 Revised: March 29, 2017 Published: April 12, 2017 A

DOI: 10.1021/acs.nanolett.7b00207 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Synthesis and characterization of BND−Co@G−MCHs. (a) Schematic of the synthesis process. (b−e) TEM images, (f) STEM image, (g−i) EDXS mapping, and (j) EELS profile of the 900 °C composite. RT, room temperature.

Synthesis and Characterization. The BND−Co@G− MCHs were synthesized as shown schematically in Figure 1a. In brief, the Co(Ac)2·4H2O was dissolved into the pyridine, followed by adding a controlled amount of 2-methylimidazole (C4H6N2) to react under magnetic stirring to form Co(mIm)2 compounds (Co2+ + 2C4H6N2 → Co(C4H5N2)2 [Co(mIm)2] + 2H+).34 The Co(mIm)2 crystals were washed by methanol and then calcined at the selected temperature of 750, 900, and 1000 °C, respectively, in argon atmosphere to obtain Co(mIm)2-derived catalysts, which self-assembled into the multiple core−shell nanocapusles. Acid-washing was applied to remove the leached metals, followed by heating under a mixture atmosphere of 70% NH3 + 30% N2 to realize the nitrogen dopant. Figure 1a shows the facile fabrication process to improve electrocatalytic properties of catalysts, including electric conductivity, catalytic active sites, and structural stability. The catalysts thus exhibit the unique physical and chemical characteristics.

of their catalytic activities has yet to be undertaken. Here we originally synthesized a uniform, MOF-derived, biphasic nitrogen-doped cobalt@graphene multiple-capsule heterostructure (BND−Co@G−MCH), acting as an effective catalyst material for Li−O2 cells, in an attempt to improve the electric conductivity, catalytic activity, and structural stability of cathode. Meanwhile, different from the traditional carbon electrodes, a flexible, stable foam-based electrode possessing a colander-like porous structure is developed. Such a nickel foam electrode improves the electric conductivity and active materials loading, and further facilitates the oxygen diffusion, catalytic reaction, and deposition of discharge products. As a result, the electrodes exhibit much improved electrochemical performance associated with unique morphologies of electrochemically grown lithium peroxides. Furthermore, we carried out the density functional theory (DFT) calculation to elucidate a profound insight into the catalytic activity and the formation mechanism of products during the discharge process. B

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nitrogen dopant becomes difficult after a higher calcination temperature. As a result, the 900 °C sample shows the structural optimization in both the crystalline and N-dopant degree. Raman spectra (Figure S4) display an intense G band and a weak D band at 1585 and 1336 cm−1, respectively, with a high IG/ID ratio, suggesting a high graphenic order of graphenes. Note that the G band is attributed to the E2g phonon that is proportional to the number of sp2-bonded carbon, whereas the D band is due to the A1g breathing mode of six-carbon rings activated by defects or curvatures.36 Additionally, a weak D′ band at 1624 cm−1 is due to the intravalley double-resonance-scattering process initiated by the presence of defects, which here are caused by the insertion of a nitrogen dopant into graphene layers.37 The XPS results further demonstrate the biphasic nitrogen doping feature. For the 900 °C sample, the fit of C 1s spectra (Figure S5) shows a main peak at 284.8 eV, which is related to the graphitic carbon (C1), suggesting that the most C atoms are arranged in the conjugated honeycomb lattices. Another two satellite peaks at 285.9 and 287.5 eV reflect different bonding structures of C−N bonds, corresponding to nitrogensp2 C (C2) and nitrogen-sp3 C (C3) bonds, respectively.36 The fit of N 1s spectra (Figure S6) are also divided into three peaks at 398.4, 399.8, and 400.9 eV, corresponding to the pyridinic (N1), pyrrolic (N2), and pyridonic N (N3) atoms, respectively. This result suggests that the nitrogen dopant brings high defects mainly in term of pyridinic, pyrrolic, and pyridonic N substitutions.36 The fit of Co 2p spectra (Figure S7) exhibits a unique feature: a sharp peak at 778.1 eV corresponds to the metallic cobalt whereas other peaks are attributed to the cobalt nitride,38,39 where the main peak of Co 2p3/2 at 780.0 eV is very close to the value for Co3+ in cobalt nitride, indicating that the dominant valence of cobalt appears to be trivalent.39 In addition, the N 1s spectra of BND−Co@G−MCHs (Figure S8) shows the chemical structural change under different calcination temperatures. Note that the intensity of peaks decreases while increasing the temperature, also indicating that the nitrogen dopant becomes difficult under higher calcination temperature. On the basis of the above observations, a biphasic N-doped Co@graphene multiple-capsule heterostructure has been fabricated and demonstrated. The DFT calculation (Figure S9) schematically shows the stable structure and composition of composite, indicating that the CoN strongly binds to defects in the graphene shell. Such nanostructures have good structural integrity since Co@CoN nanoparticles are wrapped by graphene layers and chemically bonded to graphene defects, which prevents them from being removed during the acid leaching. In addition, in such MOF-derived catalysts, active sites are uniformly distributed throughout the surface and there is no distinction between the catalyst and support regions. Therefore, maximizing the specific surface area (SSA) is equivalent to maximizing the catalytic active area. As shown in Figure S10, high SAAs of 621 and 780 m2 g−1 with the average pore sizes of 40 and 22 nm were measured for the 750 and 900 °C samples, respectively. The results reveal that such catalysts have abundant mesoporous structures, which are thus considered as an important factor contributing to the catalytic activity. Electrochemical Performance. To evaluate the electrochemical performance in Li−O2 cells, the working electrodes were prepared by mixing active materials with polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidinone (NMP) to form a slurry, which was then pasted onto carbon fibers and

The multiple-capsule structures in BND−Co@G−MCHs are investigated by transmission electron microscopy (TEM). Figure 1b shows the lotus shower-like structure of composite, where dark Co nanoparticles, sized of 10−20 nm, evenly embedded into a gentle porous carbon matrix. Figure 1c shows a single Co nanoparticle with the diameter of around 20 nm wrapped by few-layer graphenes. In Figure 1d,e, the Co@ graphene nanoparticles exhibit a core−shell shape, where crystal Co cores display highly ordered lattice spaced by 0.20 nm, which corresponds to the (111) plane of Co. A contour coating of about 3 to 8 graphene layers is observed surrounding the Co core to form a capsule nanostructure. Note that some defects on graphenes are formed due to the nitrogen dopant into carbon frameworks. The capsules (labeled by yellow numbers) connect with each other through carbon networks to construct a multiple-nanocapsule composite. We therefore consider them as multiple-capsule heterostructures. It is noteworthy that there are few layers of specific lattice fringes with 0.25 nm lattice spacing existing on the surface of Co particles, which are indexed to be cobalt nitride that are probably generated from the nitrogen dopant into surficial lattices of Co crystalline. Figure 1f shows that composite particles actually have a lotus shower-like multiple-capsule structure, associated with a uniform second particle diameter of approximately 350 nm. The composition of composite are confirmed by the energy dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS). The EDXS mapping (Figure 1g−i) shows the intense Co signal in core sites and uniform C and N distribution throughout entire composite, indicating the uniform coverage of N-doped graphene over Co nanoparticles. The EELS profile (Figure 1j) further confirms the presence of C, N, and Co elements in the composite. The structural evolution from Co(mIm)2 to BND−Co@G− MCHs was investigated by the thermogravimetry (TG) and Xray diffraction (XRD) measurements (Figures S1 and S2, Supporting Information). The TG curve (Figure S1) shows that the majority of weight loss of Co(mIm)2 occurs between 450 and 750 °C due to the decomposition of organic ligands. The residue has no obvious weight loss upon further heating to 1000 °C, indicating no undecomposed organic ligands left in the final products. High-energy XRD patterns (Figure S2) show that the Co(mIm)2 has a tetrahedral structure because each cobalt center coordinates to four nitrogen atoms of four discrete ligands.34 The first-step calcined Co(mIm)2 in argon shows the typical diffraction peaks of Co@graphene, where a sharp peak at 26.2° corresponds to the (002) plane of graphenes; whereas three strong peaks at 42.2°, 51.4°, and 75.8° are due to the cubic phase of metallic cobalt. For the final product, BND−Co@G−MCH, except for peaks of Co@ graphene it exhibits the other three peaks at 36.7°, 61.0°, and 72.7°, which are attributed to the cobalt nitride.35 This result confirms that the nitrogen indeed incorporates into some of cobalt lattices, forming the cobalt nitride. The structure and composition of BND−Co@G−MCHs are further characterized by the XRD, Brunauer−Emmett−Teller (BET), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) measurements (Figures.S3−S10). In Figure S3, XRD patterns show the structural comparison of BND−Co@G−MCHs under different calcination temperatures. Notably, the crystalline of both graphenes and cobalt crystal increase while increasing the first-step’s calcination temperature, but the opposite is true for the cobalt nitride, suggesting that the C

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Figure 2. Electrochemical performance of BND−Co@G−MCHs. (a−d) Voltage profiles versus selected cycles of (a) 750 °C sample, (b) 900 °C sample, and (c) 1000 °C sample pasted onto carbon fibers, (d) 900 °C sample pasted on the nickel foam. All four of these cells were cycled at a current density of 0.1 mA cm−2 for 10 h. (e) Voltage profiles of 900 °C sample at different current densities with a fixed capacity of 1.0 mAh cm−2. (f) Initial discharge curves of Li−O2 cells discharged to 2.2 V at a current density of 0.1 mA cm−2.

a high charge−discharge overpotential of 1.58 V for the first cycle, and the cell can hardly survive to the fifth cycle. However, the catalyst electrodes display much improved electrochemical performance as evidenced by the reduced overpotential and improved cycle-life. For the carbon fiber-based electrodes, the 750 °C sample (Figure 2a) shows a stable initial discharge plateau of ∼2.75 V and a subsequent charge potential (defined as the potential at half capacity) of ∼4.0 V. But the discharge plateau drops ∼0.13 V while the charge potential rises ∼0.08 V at the fifth cycle. After 10 cycles, the discharge further drops to 2.25 V and the capacity begins to decay. Comparatively, the 900 °C sample (Figure 2b) exhibits stable discharge plateaus with much lower potential drop. The first discharge plateau is ∼2.80 V, then it remains at ∼2.72 V even after 30 cycles. Particularly, the first charge curve of 900 °C sample shows two obvious plateaus: the first plateau at ∼3.5 V with 30% capacity is associated with the oxidation of highly conducting LiO2-like phase; a following slope with 10% capacity corresponds to the oxidation of amorphous Li2O2 shells; and the last plateau at ∼3.9 V with 60% capacity is attributed to the oxidation of remaining Li2O2 cores in parallel to some side reactions (e.g., oxidation of Li2CO3). Similar charge profiles and structural evolutions were also reported by other researchers.35,40,41 The subsequent charge profiles show high overlaps with a charge plateau of ∼3.9 V, which indicates a good cycle stability. For the 1000 °C sample (Figure 2c), it also shows a good cycling stability within 30 cycles, but its charge potential rises to 4.1 V, which is higher than that of 900 °C sample. On the basis of these results, we conclude that the higher crystallinity of catalysts ensures better electrochemical stability, and the cobalt nitride compound initiates the low charge potential.

dried under vacuum. Nickel foam electrodes were fabricated by using an infiltration method for comparison. The typical mass loading on the carbon fiber was ∼1.0 mg cm−2, while the loading on the nickel foam was ∼2.0 mg cm−2. The morphologies of composites and their electrodes were observed by the scanning electron microscopy (SEM) (Figures S11−S13). The powder samples (Figure S11) display the uniform nanoparticles. Comparatively, the higher calcination temperature of 900 °C brings more regular and uniform particles due to the higher carbonization of precursors. The carbon fiber electrodes (Figure S12) show cross-linked structures where active nanoparticles are coated onto carbon fibers. Generally, the mass loading on carbon fiber is limited by its dense fiber-networks, because enabling high mass loading may block the pore of the electrode, possibly resulting in a low catalytic efficiency. To address this problem, the processed nickel foam with more abundant channels was designed as the mass carrier and current collector. Such nickel foam electrodes (Figure S13) exhibit looser porous networks, where active materials can adhere onto nickel frameworks and connect with each other to form colanders throughout the whole electrode. These colander-like networks can provide more active areas, thus improving the catalytic efficiency. Moreover, they can fundamentally enable electronic and ionic transport as well as improve oxygen diffusion even in a high mass loading. The infiltration method for preparing foam electrode makes the high mass loading possible. Figure 2 show their voltage profiles during discharge−charge processes. For the cycling test, the cells were cycled at a current density of 0.1 mA cm−2 in 2.2−4.5 V versus Li+/Li with a fixed capacity of 1.0 mAh cm−2. For comparison, the bare carbon fiber (Figure S14) was tested as cathode in Li−O2 cell. It shows D

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Figure 3. Structural characterization of discharged products grown on carbon fiber electrode. (a−d) SEM images of the first discharged products grown on the 750 °C (a,b) and 900 °C (c,d) samples. (e,f) SEM images of the discharged products grown on the 900 °C sample after 10 (e) and 30 (f) cycles. (g) XRD patterns and (h) Raman spectra of the fresh and the first discharged electrodes. (i) Raman spectra of the 900 °C sample after selected cycles.

capability: the discharge plateau keeps above 2.7 V, and charge potential stays below 4.1 V, even while being operated at a high current density of 0.3 mA cm−2. In order to investigate the discharge capacities of these materials in Li−O2 cells, voltage profiles of the first discharge to 2.2 V at a current density of 0.1 mA cm−2 were record for comparison (Figure 2f). Among these samples, the 900 °C sample delivers the highest capacity of 3.63 mAh cm−2, while other two samples show the capacities of 3.15 mAh cm−2 (750 °C) and 3.32 mAh cm−2 (1000 °C). The result also suggests that the 900 °C sample has the best electrochemical performance. Moreover, the nickel foam electrode achieves a higher capacity of 5.98 mAh cm−2 than the carbon fiber electrode, mainly due to the higher mass loading and faster charge transfer kinetics. Characterization of Discharge Products. The discharged products were examined by the SEM, XRD, and Raman spectroscopy. As shown in Figure 3a−d, SEM images of discharged carbon fiber electrodes (to 2.2 V) exhibit unique disc-shaped products rather than previous reported toroidshaped morphologies. It is obvious that the dense and uniform disc products, sized at ∼2.5 um (diameter) and ∼0.5 um

The cycling performance of a Li−O2 cell is heavily dependent on stability of the metallic lithium anode due to the oxygen crossover effect. This is the exact case in our current work, where the lithium metal is found to completely convert to LiOH after 30 cycles, (Figure S15a). With this in mind, we disassembled a cell after 30 cycles and then reassembled the cycled catalyst cathode with a fresh Li metal anode into a new cell. This freshly built cell can run for another 30 cycles (Figure S15b). Clearly, the cycle performance in the current work was limited by the lithium metal, while the electrocatalyst maintained its good structural stability and catalytic activity during electrochemical reactions. For the nickel foam electrodes, similarly the 900 °C sample (Figure 2d) shows the same discharge/charge feature to that on carbon fiber electrode. However, a much longer plateau for the first region with 60% capacity is observed in its initial charge process, which is mainly attributed to its higher SAA and faster electric conductivity. The nickel foam electrode also shows a very stable cycling performance, indicating a promising potential for the practical application in Li−O2 cells. In addition, the 900 °C sample (Figure 2e) exhibits good rate E

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Figure 4. Density functional calculations and electrochemical mechanism study. (a) Schematic of the biphasic N-doped cobalt@graphene multiplecapsule heterostructures. (b) Schematic of the proposed mechanism for reactions occurring during the discharge process. (c−f) Density functional theory (DFT) calculations showing the binding energies between Li2O2 monomer and active sites on the support catalyst surfaces. The pyridinic (c) and (d) pyrrolic (d) N-doping sites on graphenes, and the CoN (111) surface with the Co (e) and N (f) terminations were taken into account for calculations.

the XRD results, indicating that the better crystalline and higher SAA of active materials facilitate the formation of Li−O products. Furthermore, Raman spectra for different cycled electrodes after discharging (Figure 3i) illustrate the chemical evolution of discharged products during cycling. Three samples after selected cycles, corresponding to SEM images in Figure 3d−f, respectively, are detected by Raman spectroscopy for comparison. As expected, the first discharged sample shows the strongest signal for Li2O2 and LiO2-like products. The 10th discharged sample shows similar bands with almost equivalent intensity to those of first discharged sample, while after 30 cycles, the discharged sample still shows three bands of LiO2like, LiO2−C, and Li2O2 products but without any Li2CO3 signals. These results indicate the good electrochemical reversibility and stable cycling performance of Li−O2 cells. In addition, high-energy XRD and TEM observations (Figures S17 and S18) for cycled electrodes demonstrate that the BND−Co@G−MCHs maintain good structural stability during the electrochemical reactions. Discussion. DFT calculations were carried out to elucidate the energetics of interactions between Li2O2 and active sites on support catalyst surfaces. Figure 4a,b illustrates active regions on catalysts and the mechanism for the nucleation of Li2O2 on active sites, respectively. In such MOF-derived catalyst, some of Co and CoN nanoparticles are exposed or not fully wrapped by graphenes due to the acid leaching. Moreover, during the calcination process especially for the second-step calcination in the NH3/N2 atmosphere, a lot of defects on graphenes are generated (TEM in Figure 2d), which allows more exposure of Co and CoN nanoparticle. These exposed Co and CoN nanoparticles together with the N-doped defects on graphene build up the active sites for the formation of Li2O2. Once the Li2O2 nanoparticles form on active sites, they can act as nucleation sites for the continuous Li2O2 growth. Figure 4c−f shows the binding strengths of Li2O2 on both the N-doped graphenes and CoN (111) surface, which are strong enough for

(thickness), grow smoothly onto the electrodes. Remarkably, the 900 °C sample (Figure 3c,d) shows a much denser and more uniform morphology associated with larger size than that of the 750 °C sample (Figures 3a,b). Actually, this difference is due to the larger SAA and higher catalytic activity of 900 °C sample. Similarly, dense and uniform disc products are also observed on nickel foam electrodes after the first discharge (Figure S16). The only difference is that nickel foam electrodes provide higher electric conductivity and more nucleation sites favorable for the formation of smaller Li2O2 particles that are easier to be decomposed than the larger ones, so its charge overpotential is lower than that of carbon fiber electrodes. During the following dozens of cycles, the discharged electrodes (to 2.2 V) still show the high-yield surface morphologies consisting of disc products, which becomes thinner and larger as the cycle number increases (Figure 3e,f). The structural evolution of discharge products was further demonstrated by the XRD and Raman spectroscopy. Figure 3g shows XRD patterns of the fresh and first discharged electrodes. The fresh electrodes show the similar diffraction peaks indexed to active materials, except for several characteristic peaks belonging to the carbon fiber (labeled by purple squares). In contrast, the discharged electrodes exhibit many diffraction peaks indexed to Li2O2 crystals (blue star), and few weak peaks for the LiO2-like species (black plum) and Li2CO3 byproducts (black circle).12,17 This result indicates that disc products are mainly composed of Li2O2 crystalline. Raman spectra for the first discharged electrodes (Figure 3h) further identify their structure and chemical composition: two intense bands at 1128 and 1510 cm−1 are the characteristic bands of LiO2-like species and LiO2−C interactions, respectively.5,11 A small band at 790 cm−1 is due to the Li2O2 crystalline,11 and another very weak band at 1040 cm−1 is ascribed to Li2CO3.7 In contrast to the 750 °C sample, the 900 °C sample shows higher band intensity of LiO2-like, LiO2−C, and Li2O2 products, but lower intensity for Li2CO3. This finding is well matched with F

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the Li2O2 deposition. The pyridinic (Figure 4c) and pyrrolic (Figure 4d) N sites on graphene have been taken into account, both of them were detected experimentally from the N 1s XPS spectra (Figure S6). Both N sites were calculated to have the strong binding interactions with Li2O2 (binding energy: −2.17 and −1.79 eV, respectively). In addition, both the Co (Figure 4e) and N (Figure 4f) terminations of CoN (111) surface show strong binding interactions with Li2O2 (binding energy: −1.54 and −1.31 eV, respectively). More calculated conformations of Li2O2 binding on the CoN (111) surface are shown in Figures S19 and S20. These calculation results indicate that both the Ndoped sites on graphene and exposed CoN areas can be critical active sites for the formation and deposition of Li2O2 products. And these strong binding interactions with Li2O2 can be responsible for the formation of unique morphology (“discshape”) of Li2O2 products. Previous studies on graphene type of materials as cathodes have shown a different morphology (“toroid-shape”) of Li2O2, which could attribute to the relative weak interactions between Li2O2 and graphene.42,43 The DFT calculation (Figure S21) shows that the Li2O2 monomer has a rather weak binding on the pristine graphene (binding energy: −0.4 eV). In conclusion, scalable and cost-effective MOF-derived catalysts for Li−O2 cells have been developed in the form of a biphasic N-doped cobalt@graphene multiple-capsule heterostructure. Compared to some recently reported nonprecious catalysts, such as metal oxides, nitrides, and sulfides, as well as pure MOF catalysts (Table S1), this heterostructured catalyst exhibits unique physical and chemical characteristics, and advanced electrochemical performance. In such special catalyst, both N-doped graphene shells and crystal Co@CoN cores are nucleated simultaneously, exhibiting the high electrical conductivity, super catalytic activity and excellent structural stability. In addition, a flexible, colander-like porous electrode configuration improves the electrochemical reaction efficiency. Indeed, such a unique structural design has several advantages: (1) the N-doping into graphenes and cobalt creates abundant active regions, and the multiple-nanocapsule configuration makes high/uniform electroactive zones possible that greatly enhance the catalytic efficiency; (2) the N-doped graphenes function as electrical highways and mechanical backbones so that all the nanoparticles are electrocatalytically active; (3) the abundant porous structures both in catalysts and electrodes facilitate the electrolyte diffusion, oxygen storage, and transportation, as well as the Li2O2 deposition. As a result, the catalyst cathodes exhibit improved electrochemical catalytic performance, including high specific capacity, low charge− discharge overpotential, good cycling stability, and unique morphology of Li2O2 products. This study exhibits a new lowcost and highly active catalyst and also provides new avenues for rational engineering of lithium−oxygen cathodes.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.). *E-mail: [email protected] (D.L.). *E-mail: [email protected] (K.A.). ORCID

Jun Lu: 0000-0003-0858-8577 Reza Shahbazian-Yassar: 0000-0002-7744-4780 Khalil Amine: 0000-0001-9206-3719 Author Contributions #

G.Q.T., L.N.C., and R.A. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy (DOE) under Contract DE-AC0206CH11357 with the support provided by the Vehicle Technologies Office, DOE, Office of Energy Efficiency and Renewable Energy. L.N.C and D.J.L acknowledge the support from the U.S. DOE, Office of Science. TEM was accomplished at the Electron Microscopy Center in the Center for Nanoscale Materials at Argonne National Laboratory, a DOE-BES Facility, under Contract No. DEAC0206CH11357. DFT calculations were supported by the U.S. DOE, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under Contract DE-AC0206CH11357. Use of Advanced Photon Source (11-ID) was supported by the U.S. DOE, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357.

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b00207. Experimental details; DFT calculations and electrochemical tests; TGA, XRD, Raman, XPS, BET, SEM, and TEM characterization of the materials (PDF) G

DOI: 10.1021/acs.nanolett.7b00207 Nano Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.nanolett.7b00207 Nano Lett. XXXX, XXX, XXX−XXX