Hierarchical hollow microspheres grafted with Co nanoparticle-embedded bamboo-like N-doped carbon nanotube bundles as ultrahigh rate and long-life cathodes for rechargeable lithium-oxygen batteries

Accepted Manuscript Hierarchical Hollow Microspheres Grafted with Co Nanoparticle-Embedded Bamboo-like N-Doped Carbon Nanotube Bundles as Ultrahigh Rate and LongLife Cathodes for Rechargeable Lithium-Oxygen Batteries Jung Hyun Kim, Seung-Keun Park, Yeon Jong Oh, Yun Chan Kang PII: DOI: Reference:

S1385-8947(17)32129-0 https://doi.org/10.1016/j.cej.2017.12.018 CEJ 18179

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

23 September 2017 21 November 2017 5 December 2017

Please cite this article as: J.H. Kim, S-K. Park, Y.J. Oh, Y. Chan Kang, Hierarchical Hollow Microspheres Grafted with Co Nanoparticle-Embedded Bamboo-like N-Doped Carbon Nanotube Bundles as Ultrahigh Rate and LongLife Cathodes for Rechargeable Lithium-Oxygen Batteries, Chemical Engineering Journal (2017), doi: https:// doi.org/10.1016/j.cej.2017.12.018

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Hierarchical Hollow Microspheres Grafted with Co NanoparticleEmbedded Bamboo-like N-Doped Carbon Nanotube Bundles as Ultrahigh Rate and Long-Life Cathodes for Rechargeable Lithium-Oxygen Batteries

Jung Hyun Kim a,b,+, Seung-Keun Park a,+, Yeon Jong Oh a, Yun Chan Kang a,*

a

Department of Materials Science and Engineering, Korea University, Anam-Dong,

Seongbuk-Gu, Seoul 136-713, Republic of Korea b

Energy & Environmental Division, Korea Institute of Ceramic Engineering & Technology

(KICET), 101 Soho-ro, Jinju-si, Gyeongsangnam-do 52581, Republic of Korea

+ These authors contributed equally to this work

*Corresponding authors. E-mail: [email protected]. Tel.: +82-2-928-3584. Fax: +82-2-3290-3268. (Yun Chan Kang)

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Abstract Rational design of efficient, affordable, and durable electrocatalysts for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is essential for rechargeable lithiumoxygen (Li–O2) batteries. We present for the first time hierarchical hollow microspheres grafted with metallic Co-embedded bamboo-like N-doped carbon nanotube bundles (Co-bNCNTs hollow microspheres) as oxygen electrodes for Li-air batteries. Hierarchical composite microspheres are prepared via a facile two-step process involving synthesis of Co3O4-MgO hollow microspheres by spray pyrolysis, followed by internal and external growth of bamboo-like NCNTs in the shells. During post-treatment, metallic Co and MgO nanoparticles play key respective roles in catalyzing in-situ growth of NCNTs and maintaining structural integrity of the composites. The hierarchical composite structure with Co and N doping not only provides ample active sites for the OER and ORR, but also sufficient space for storing produced Li2O2. Thus, Co-b-NCNTs hollow microspheres exhibit high initial round-trip efficiency, long-term cycling and ultrahigh rate performances when applied as oxygen electrodes for Li–O2 batteries. The initial discharge capacity and round-trip efficiency at a current density of 200 mA g-1 are 28968 mA h g-1 and 78.2%, respectively. Specific capacities at cutoff capacities of 500 and 1000 mA h g-1 are stable for 201 and 157 cycles, respectively. Keywords: Li-O2 batteries, carbon nanotubes, hierarchical structure, cathode material, nanostructured material, spray pyrolysis

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1. Introduction Rechargeable lithium-oxygen (Li-O2) batteries have recently attracted tremendous attention due to their extremely high energy density (~3500 W h kg-1), which is ~8 times larger than that of current Li-ion batteries [1-5]. However, progress in the utilization of Li-O2 batteries has been hampered by factors including their large polarization, poor cycle life, and low round-trip efficiency [6-10]. These problems are mainly attributed to the sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) occurring at the oxygen electrode [11-18]. Therefore, the electrochemical performance of Li-O2 batteries is strongly influenced by both the architecture and materials of the oxygen electrode. Recently, transition metal nanoparticles encapsulated by carbon nanotubes (CNTs) have attracted broad interest and are emerging as promising nanocatalysts for the OER and ORR [19-22]. In contrast to the traditional belief that encapsulation by graphitic carbon shells would render such metal catalysts inert, these materials exhibit excellent activities toward several electrocatalytic reactions due to the synergistic effects arising from the intimate contact between the N-doped graphitic layer and transition metal [22, 23]. Lin et al. synthesized metallic Co embedded within N-doped carbon nanotubes as a bifunctional catalyst to promote the OER and ORR. When tested as oxygen electrode materials for Zn-air batteries, these hybrids exhibited a high open-circuit voltage of 1.45 V and stability over 12 h in alkaline condition [23]. Bao et al. reported the facile synthesis of Fe nanoparticles encapsulated within N-doped carbon nanoshells by pyrolysis of dicyandiamide and Fe precursor. The resulting Fe@N-doped carbon nanoshell material exhibits improved bifunctionality for OER and ORR in alkaline condition compared to commercial Pt/C and IrO2 [20]. However, most of these materials have been restricted to one-dimensional (1D) structures with limited gas and electrolyte diffusion channels and low storage for discharge

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products including Li2O2. Therefore, it is highly desirable to develop a uniquely designed nanocatalyst to overcome these problems. Hierarchical three-dimensional (3D) nanostructures composed of low-dimensional segments are considered to be a new type of promising catalysts, since these materials may inherit the intrinsic electrochemical properties of their building blocks and provide unexpected advantages as electrocatalysts [24-30]. Among the potential building blocks, 1D nanomaterials are particularly promising candidates owing to their higher structural stability and efficient electron diffusion properties [31-33]. However, the synthesis of hierarchical 3D nanostructures composed of 1D structured building blocks as electrocatalysts for the oxygen electrode of Li-O2 batteries remains a significant challenge. Herein, we report for the first time the successful synthesis of hierarchical hollow microspheres grafted with Co nanoparticle-embedded bamboo-like N-doped CNT bundles (Co-b-NCNTs hollow microsphere) via a facile two-step process. This process involved the preparation of Co3O4-MgO hollow microspheres via one-pot spray pyrolysis, followed by internal and external growth of bamboo-like N-doped CNT bundles in the shells; during this step, metallic Co nanoparticles were encapsulated in N-doped graphitic carbon layers. The metallic Co and MgO nanoparticles play respective roles as a nanocatalyst for the formation of bamboo-like N-doped CNTs and as a template to maintain structural integrity during hightemperature thermal treatment. This uniquely designed material possesses some advantages as the oxygen electrode for Li-O2 batteries. Firstly, the hierarchical hollow structures not only facilitate oxygen diffusion and electrolyte penetration, but also provide sufficient space for storing the produced Li2O2. Secondly, the intimate contact between the metallic Co nanoparticles and the bamboo-like N-doped CNTs could improve the electrocatalytic activities for the ORR and OER. Thirdly, the bamboo-like N-doped CNTs (b-NCNTs) firmly

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grafted to hollow microspheres improve the electronic conductivity of the electrode and act as a physical buffer to strengthen the structural integrity.

2. Experimental section Sample preparation: The Co3O4-MgO hollow microspheres were synthesized via a facile one-pot spray pyrolysis process, which was well described in our previous articles [34-36]. The spray solution was prepared by dissolving cobalt (II) nitrate hexahydrate (0.1 M) and magnesium (II) nitrate hexahydrate (0.1 M) in distilled water (500 mL). The reactor was maintained at a temperature of 600 °C. To obtain the Co-b-NCNTs hollow microspheres, a small rectangular-shaped alumina boat containing the obtained Co3O4-MgO hollow microspheres was loaded into a larger alumina boat with a cover. Dicyandiamide powder was placed around the small alumina boat as a carbon and nitrogen source. During thermal treatment under Ar atmosphere, the Co3O4-MgO hollow microsphere powder was firstly reduced at 400 °C for 3 h and then 1D structured bamboo nanotubes were grown at 800 °C for 1 h, with ramp rates of 2 and 5 °C min-1, respectively. HCl etching of MgO produced the Co-loaded and bamboo-like NCNTs microspheres (Co-b-NCNTs hollow microsphere). For comparison, Co3O4 hollow microspheres were also prepared by spray pyrolysis under the same conditions, but without Mg precursor in the spray solution. The Co-NCNTs were formed from the Co 3O4 hollow microspheres by post-treatment with dicyandiamide. Characterization: The morphologies and structures of the prepared samples were examined using scanning electron microscopy (SEM, VEGA3 SBH) and transmission electron microscopy (TEM, JEM-2100F). The crystal structures were investigated by X-ray diffraction (XRD, X’Pert PRO), with Cu Kα radiation (λ = 1.5418 Å) at the Korea Basic Science Institute (Daegu). X-ray photoelectron spectroscopy (XPS) was performed with Thermo Scientific K-Alpha, using Al Kα radiation to analyze the chemical compositions of 5

the specimens. The XPS spectra were acquired by calibration based on a value of C 1s (284.5 eV). The surface areas of the samples were measured using the Brunauer–Emmett–Teller (BET) method, with N2 as the adsorbate gas. To determine the carbon content in the samples, thermogravimetric (TG) analysis was performed in air using Pyris 1 TGA (Perkin Elmer) over a temperature range of 25–800 °C, at a heating rate of 10 °C min-1. The crystallinity analysis of the carbon in the samples was conducted by Raman spectroscopy (Jobin Yvon LabRam HR800, excited by a 632.8-nm He/Ne laser) at room temperature. ORR and OER tests: The microspheres (5 mg) were dispersed in 1 mL of a binary mixture of isopropyl alcohol and deionized (DI) water (1:4 v/v), with Nafion (100 µL, 5 wt. %, Aldrich), by sonication for at least 30 min to form a homogeneous ink. A droplet (10 µL) of the ink was then released onto a glassy carbon electrode (GCE; 5 mm in diameter) to obtain a modified GCE. Electrochemical measurements were performed in a standard three-electrode configuration, using a rotating disk electrode (RDE, AFMSRCE, Pine Research Instrumentation) with a rotation speed of 1600 rpm connected to a potentiostat (ZIVE SP1, WonA Tech). A graphite rod and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. The electrocatalytic activity of the catalysts was examined via linear sweep voltammetry (LSV) in an O2-purged 0.1 M KOH solution at 298 K. The ORR and OER polarization curves were tested in the potential range of 0.0 to -0.6 V and 0.0 to 1.0 V vs. SCE, respectively, at a rate of 10 mV s−1. In addition, the electrochemical impedance over a frequency range of 0.01 Hz-100 kHz was measured via electrochemical impedance spectroscopy (EIS). Li–O2 battery test: The electrochemical performance of the Li–O2 batteries was tested using a coin-type cell (CR2032). The positive side has uniform small holes, with a diameter of 1 mm, to enable oxygen flow. Catalyst (90 wt. %) and polyvinylidene fluoride (PVDF, 10 wt. %) binder were homogeneously mixed in a N-methyl-2-pyrrolidone (NMP) solution. Carbon 6

paper with a diameter of 14 mm was then uniformly coated with the mixture to produce the air cathode. After coating, the carbon paper was dried in a vacuum-drying oven at 80 °C for 12 h to remove the residual solvent. The active material mass loading was 0.3 mg cm-2. The entire cell assembly of the lithium–air battery was carried out in a glove box under argon atmosphere; the water and oxygen contents were lower than 0.1 ppm. A metallic lithium foil and a glass filter (Whatman) were used as the counter electrode and separator, respectively. The electrolyte comprised 0.5 M bis(trifluoromethane)sulfonimide lithium salt (LiTFSI), 0.5 M LiNO3 and 0.05 M LiI dissolved in tetraethylene glycol dimethyl ether (TEGDME). The separator was fully soaked with electrolyte and the cells were relaxed for 1 h before testing. The discharge/charge characteristics of the samples were investigated by cycling in the 2.0– 4.35 V potential range at various current densities in a pure O2 atmosphere. The capacity was calculated based on the mass of the catalyst loaded on the cathode.

3. Results and discussion The formation mechanism of the hierarchical structured Co-b-NCNTs hollow microsphere is described in Fig. 1a. In the first step, the Co 3O4-MgO hollow microspheres were prepared from droplets containing both Co(NO 3)2 and Mg(NO 3)2 (Fig. 1a-1 and 2). Post-treatment with dicyandiamide in a reducing atmosphere produced Co-MgO hollow microspheres, in which the metallic Co nanocatalysts were uniformly distributed over the hollow MgO matrix (Fig. 1a-3). Subsequently, N-doped and bamboo-like CNTs were grown on the internal and external surfaces of the CoMgO hollow microspheres using CH x and NH3 gases formed by decomposition of dicyandiamide (Fig. 1a-4). HCl etching of the MgO hollow matrix produced Co-bNCNTs microspheres. The forest of b-NCNTs constructed on the internal and external surfaces of the hollow spheres provided space for the Li 2O2, which is produced when 7

Co-b-NCNTs hollow microsphere is applied as the cathode material for Li-air batteries (Fig. 1b).The Co-loaded and N-doped CNTs with high electrical conductivity acted as excellent ORR and OER catalysts during the discharge and charge processes, respectively. The morphologies of the Co3O4-MgO hollow microspheres prepared by one-pot spray pyrolysis are shown in Fig. 2. The SEM and low-resolution TEM images shown in Fig. 2a-c reveal the hollow morphology of the microspheres, produced as a result of the rapid droplet drying rate. The composite microspheres have holes formed by emission of pressured gas, as shown by the arrows in Fig. 2a. The holes play a key role in enabling penetration of liquid electrolyte into the hierarchical structured Co-bNCNTs hollow microsphere. The high-resolution TEM image shown in Fig. 2d reveals the presence of ultrafine Co 3O4 and MgO nanocrystals. The XRD and selected area electron diffraction (SAED) patterns shown in Fig. S1 and 2e, respectively, confirm the formation of composite microspheres with a mixed crystal structure consisting of (220) and (311) of Co 3O4, and (200) and (220) of MgO phases. The elemental mapping images shown in Fig. 2f reveal the formation of composite microspheres with a uniform distribution of Co 3O4 and MgO nanocrystals. The morphologies of the hierarchical structured Co-b-NCNTs hollow microsphere are shown in Fig. 3. The SEM and low-resolution TEM images shown in Fig. 3a-c reveal that the microspheres consist of well-grown NCNTs. The spherical morphology of the Co3O4-MgO microspheres is maintained even after NCNT growth and etching of the MgO matrix. In the high-resolution TEM images (Fig. 3d), the encapsulated Co nanocrystals observed at the end tip of the NCNTs, confirm the growth of CNTs over the Co nanocatalyst. Also, NCNTs have the bamboo-like morphology, which was attributed to presence of pentagon structures in the graphite network due to N-doping [20, 37]. To investigate the role of metallic Co for the 8

growth of CNTs, hollow MgO microspheres prepared by spray pyrolysis were treated with DCDA under the same condition as hollow Co 3O4-MgO microspheres (Fig. S2). However, no NCNTs bundles on the surface of the microspheres were found, suggesting that metallic Co nanoparticles play key role in catalyzing in-situ growth of NCNTs. The XRD and SAED patterns shown in Fig. S1 and 3e, respectively, reveal the coexistence of metallic Co nanocatalyst and highly graphitized NCNTs. The uniform distribution of metallic Co nanocatalysts and NCNTs across the microspheres in the elemental mapping images (Fig. 3f) confirms the uniform formation of bamboo-like NCNTs on the internal and external surfaces of the hollow microspheres. The elemental mapping images also confirm the complete etching of MgO and presence of N-doping. The XPS survey spectrum of the hierarchical structured Co-b-NCNTs hollow microsphere, shown in Fig. 4a, confirms the presence of Co, C, N, and O. The deconvoluted C 1s spectrum (Fig. 4b) shows peaks corresponding to C-C (sp2 bonded carbon), C-N/C-C (sp3), and -COO components at 284.5, 285.6, and 288.7 eV, respectively. The N 1s spectrum shown in Fig. 4c reveals the presence of three types of N species; namely, pyridinic N at ~397.9 eV, pyrrolic N at ~400.1 eV, and graphitic N at ~400.9 eV, accounting for 45.4, 33.3, and 21.3 % of the N configurations, respectively [38-41]. Recently, some studies have shown that a high proportion of pyridinic N and graphitic N could increase current density and the density of π states of the carbon atoms near the Fermi level, thus facilitating O 2 reduction [42, 43]. Thus, the high pyridinic N and graphitic N content in the hierarchical structured Co-bNCNTs hollow microsphere probably results in enhanced intrinsic ORR activity. The Co 2p spectrum shown in Fig. 3d reveals two major peaks at 779.2 and 794.8 eV, corresponding to Co 2p 3/2 and Co 2p1/2, respectively. The deconvoluted Co 2p spectrum displays characteristic peaks corresponding to Co 0, Co2+, and satellites. The 9

Co-b-NCNTs hollow microsphere composite possesses partial Co 2+ species owing to the existence of the CoO phase in the composite formed by partial oxidation of metallic Co nanocrystals. The morphologies of the materials synthesized as comparison samples are shown in Fig. S3 and 5. Hollow-structured Co3O4 microspheres are prepared by one-pot spray pyrolysis, as confirmed by the TEM images shown in Fig. S3a and b. The SAED pattern shown in Fig. S3c confirms the cubic phase-pure Co3O4 microspheres. The Co3O4 microspheres transform into the non-spherical Co-NCNTs after CNT growth, as confirmed by the SEM and TEM images shown in Fig. 5a-c. The TEM image shown in Fig. 5c confirms the formation of hollow NCNTs with wrinkled walls in which the encapsulated Co nanocrystals are larger than that of Co-b-NCNTs hollow microsphere. These results clearly show the role of MgO matrix; it could confine the size of metallic Co nanocrystals and maintain structural integrity during post-treatment. The elemental mapping images shown in Fig. 5f confirm the growth of Ndoped CNTs by metallic Co nanocatalyst. The N2 adsorption and desorption isotherms and Barrett-Joyner-Halenda (BJH) pore-size distributions of the Co-b-NCNTs hollow microsphere and Co-NCNTs are shown in Fig. S4. The Co-b-NCNTs hollow microsphere have well-developed mesopores due to the internal spaces between the well-grown NCNTs. The BET surface areas of the Co-b-NCNTs hollow microsphere and Co-NCNTs are 144 and 51 m2 g-1, respectively. The TG curves of the two samples are shown in Fig. S5a. The NCNTs in Co-b-NCNTs hollow microsphere and Co-NCNTs combust at temperatures of 450 and 350 °C, respectively. The formation of highly crystallized CNTs with small widths increases the BET surface area and the combustion temperature of CNTs in Co-b-NCNTs hollow microsphere. The weight losses for Co-b-NCNTs hollow microsphere and Co-NCNTs at temperatures below 700 °C are 82.3 and 74.3 %, respectively, and the calculated NCNT contents are 86.9 and 81.1 %, respectively. 10

The structural features of carbon in composites was investigated by Raman spectroscopy. Fig. S5b showed two major peaks located at ~1360 and ~1590 cm-1, which correspond to the D and G bands of graphitic carbon, respectively [44,45]. The peak intensity ratios of D to G bands (ID/IG), which is commonly used as a measure of graphitization degree, were calculated as approximately 1.08 for Co-b-NCNTs hollow microsphere and 0.97 for Co-NCNTs. These results suggest that the graphitic carbon of Co-b-NCNTs hollowmicrosphere had more defects than that of Co-NCNTs [46,47]. The ability of the hierarchical structured Co-b-NCNTs hollow microspheres to catalyze both the ORR and the OER is compared to that of the Co-MgO hollow microsphere and CoNCNTs, as shown in Fig. 6. The RDE linear sweep voltammograms in O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm are compared in Fig. 6a. Commercial Pt-C nanocatalyst was the most active material for ORR with small onset overpotential. Co-bNCNTs hollow microspheres and Co-NCNTs have lower onset overpotentials than that of the Co-MgO catalyst. Co-embedded bamboo-like N-doped carbon nanotube shows superior ORR activities compared to those of the Co nanocatalyst. The onset overpotentials of Co-b-NCNTs hollow microspheres and Co-NCNTs are -168 and -172 mV vs. SCE, respectively, which are lower than those of the N-doped and undoped CNTs reported in the literature [48-50]. The half-wave potentials (E1/2) of the Co-b-NCNTs hollow microspheres and Co-NCNTs are -220 and -230 mV vs. SCE, respectively, and their limiting current density values are 4.4 and 3.4 mA cm-2, respectively. The high limiting current density of Co-b-NCNTs hollow microspheres reveals the existence of a large number of active sites due to the formation of many N-doped CNTs with small widths. The ORR properties such as onset potential, halfwave potential and limiting current density of Co-b-NCNTs hollow microspheres have been compared to those of cobalt based cathode materials for Li-O2 batteries reported in the 11

previous literatures. A summary presented in Table S1 show the excellent ORR properties of Co-b-NCNTs hollow microspheres. Its excellent ORR performance is probably because of its large surface area, high N content and features with Co particles embedded in N-doped carbon nanotubes [51]. In addition, embedded transition-metal nanoparticles in N-doped CNTs can lower the local work function of the carbon surface and, thereby, enhance the ORR activity [52]. Another important parameter for characterizing the performance of ORR is the four-electron selectivity of the catalyst because the direct reduction of oxygen to water or to OH- by the four- electron process is more efficient than the reduction of hydrogen peroxide via a two-electron process. RDE tests were carried out in O2-saturated 0.1 M KOH solution at a scan rate of 10 mVs-1, and the number of electrons involved in the oxygen reduction reaction were calculated from the Koutecky-Levich (K-L) equation [53]. Fig. S6 displays LSV curves for Co-b-NCNTs and corresponding K-L plots with the inverse current density (j1

) versus the inverse of the square root of the rotation speed (-1/2) at different potentials. The

linearity of K-L plots and the near parallelism of fitting lines indicate the first-order reaction kinetics toward the concentration of dissolved oxygen and the similar electron transfer number for the ORR at different potentials [54]. Based on the slopes of K-L plots, the electron transfer numbers were calculated to be ~3.8, ~3.9 and ~3.9 at -0.6, -0.5 and -0.4 V vs. SCE for Co-b-NCNTs, which suggest a direct reduction of O2 to H2O via the 4-electron pathway. OER properties from LSV polarization curves of the four catalysts were recorded during an anodic scan (at 10 mV s -1) between 0.0 and 1.0 V vs. SCE (Fig. 6b). OER onset potential of Co-b-NCNTs hollow microspheres was more positive than that of the commercial RuO2, and their overpotential value at a current density of 10 mA cm-2 was similar to that of the RuO2. The Co-b-NCNTs hollow microspheres show the highest limiting diffusion current (27.3 mA cm-2), which is significantly higher than that of the Co-NCNTs (21.9 mA cm-2) and Co-MgO hollow microspheres (4.7 mA cm-2), and commercial RuO2 12

(21.8 mA cm-2). The bifunctional performance can be quantified by the potential difference between ORR and OER (ΔE = Ej=10 − E1/2, where Ej=10 represents the potential at a current density of 10 mA cm-2). The Co-b-NCNTs hollow microspheres exhibit the highest bifunctional catalytic performance (ΔE = 1.08 V). Consequently, the Co-b-NCNTs hollow microspheres show the best ORR and OER electrocatalytic activities. Pyridinic N has a beneficial effect in promoting the ORR and OER catalytic activities of the Co-b-NCNTs hollow microspheres. On the other hand, the hollow-structured Co-MgO hollow microspheres composed of open ultrafine Co nanocatalysts show poor ORR and OER electrocatalytic activities due to their low electrical conductivity. The initial discharge and charge profiles of the Li–O2 batteries with the prepared Co-bNCNTs, Co-MgO hollow microspheres, and Co-NCNTs at a fixed capacity of 500 mA h g-1 and a current density of 200 mA g-1 are shown in Fig. 7a. The Li–O2 batteries with the Co-bNCNTs hollow microspheres, Co-NCNTs and Co-MgO hollow microspheres show initial discharge overpotentials of 0.22, 0.27, and 0.29 V, respectively, and initial charge overpotentials of 0.24, 0.42, and 0.46 V, respectively. The low overpotential of the electrode containing Co-b-NCNTs hollow microspheres demonstrates the superior catalytic activity of the uniquely structured composite in the discharging and charging processes. The round-trip efficiencies of Co-b-NCNTs hollow microspheres, Co-NCNTs, and Co-MgO microspheres are 86, 80, and 78 %, respectively. The synergistic effect between N-doping and the hierarchical structure of Co-b-NCNTs hollow microsphere improves the electrocatalytic activities towards the OER and ORR, resulting in higher electrochemical performance for LiO2 batteries. The cycling stability of Li–O2 batteries with the three cathode materials at a current density of 200 mA g-1 and cutoff capacities of 500 and 1000 mA h g-1 are shown in Fig. 7b and c, respectively. At both cutoff capacities, Co-b-NCNTs hollow microspheres exhibit superior 13

cycling performances compared to those of the other two samples. At a cutoff capacity of 500 mA h g-1, the specific capacities of Co-b-NCNTs hollow microspheres are stable for 201 cycles. In contrast, the specific capacities of the prepared Co-NCNTs and Co-MgO hollow microspheres dropped from 131 and 87 cycles, respectively. At a cutoff capacity of 1000 mA h g-1, the specific capacities of Co-b-NCNTs hollow microspheres, Co-NCNTs, and Co-MgO hollow microspheres decreased after 157, 74, and 45 cycles, respectively. At a cutoff capacity of 500 mA h g-1, the terminal charge voltages of Co-b-NCNTs microspheres (Fig. 7d) increase slightly during 201 cycles, which implies that only a small change in the internal resistance occurs during cycling due to the stability of the catalytic active sites. However, the terminal charge voltages of Co-NCNTs and Co-MgO hollow microspheres at a cutoff capacity of 500 mA h g-1 noticeably increase after 16 and 9 cycles, respectively, to 4 V. The gradual increase in overpotential and limited cycle performance indicates that the reaction products have not yet fully decomposed under the test conditions. The morphologies of the three samples obtained after the 1st discharge and 1st charge in the potential range of 2.0–4.35 V are shown in Fig. S7. The ultrafine Li2O2 product particles deposited inside and outside the Co-b-NCNTs hollow microspheres during the first discharge process are clearly observable in Fig. S7a. After the first charge process, Li2O2 product particles are not observed in Co-b-NCNTs hollow microspheres. The Li2O2 product particles formed during the discharge process decompose completely during the following charge process. However, the SEM images of the cathodes with Co-NCNTs and Co-MgO hollow microspheres obtained after the initial discharge and charge processes reveal that the Li 2O2 product is deposited in the particles, indicated by arrows in Fig. S7b and c. Despite the similar impedances of the fresh cells for the three samples, shown in Fig. S8, the Co-bNCNTs hollow microspheres show the highest calaytic activity due to their unique, hollow morphological structure and numerous highly dispersed C-Nx active sites, which improve the formation and decomposition of the Li2O2 product. The morphologies of the three samples 14

obtained after the 30th fully charged state at a cutoff capacity of 500 mA h g-1 are shown in Fig. 8. The hierarchical structure of Co-b-NCNTs hollow microspheres is well maintained even after the long-term cycling test (Fig. 8a). In addition, even after 30 cycles, the Li2O2 product particles are scarcely observable within Co-b-NCNTs hollow microspheres, which indicates that the Li2O2 products are reversibly decomposed to Li ions and oxygen species during cycling. Raman spectrum of Co-b-NCNTs cathode obtained after 30 cycles is shown in Fig. S9. The spectrum consists of graphitic G (not shown) and D bands from the carbon paper. The peaks at wavenumbers of 1088, 748 and 712 cm-1 associated with the presence of Li2CO3 are not observed. In general, unexpected products such as Li2 CO3, generated by side reactions between electrolyte and the catalysts, are accumulated during cycling. In this study, however, the Li2CO3 product was completely decomposed during charge process. This further demonstrates that Co-b-NCNTs hollow microspheres are the efficient catalysts for the OER. On the other hand, Li2O2 products deposited over Co-NCNTs and Co-MgO hollow microspheres are observed, as indicated by arrows in Fig. 8b and c. The initial discharge and charge curves of Co-b-NCNTs hollow microspheres in the voltage range of 4.35–2.0 V vs. Li/Li+ at various current densities are shown in Fig. 9. Co-b-NCNTs hollow microspheres show superior rate performances; the capacity decreases slightly when the current density increases from 200 to 2000 mA g-1. The initial discharge capacities at current densities of 200, 500, 1000, and 2000 mA g-1 are 28968, 22103, 15633, and 10613 mA h g-1, respectively, and the round-trip efficiencies are 78.2, 75.1, 71.4, and 69.0 %, respectively. The morphologies of the cathode with Co-b-NCNTs hollow microspheres obtained after initial discharge and charge processes at various current densities are shown in Fig. 9b-d. The ultrafine Li2O2 product particles deposited inside and outside Co-b-NCNTs hollow microspheres during the first discharge process are clearly observed, irrespective of the current density. However, Li2O2 product particles are not observed in Co-b-NCNTs 15

hollow microspheres after the first charge process, irrespective of the current density, as confirmed by the SEM images. The Li2O2 product particles formed during the discharge process decompose completely during the following charge process, irrespective of the current density. The characteristics of Co-b-NCNTs hollow microspheres have been compared to those of Co-based cathode materials for Li-O2 batteries reported in the literature and summarized in Table S2.

Co-b-NCNTs hollow microspheres have superior capacities,

long-term cycling and excellent rate performances compared to those of the Co-based materials reported in the literature. The synergetic effect of the presence of many highly active sites in the N-doped CNTs with small widths and the porous and hierarchical structure enabling uniform deposition of the Li2O2 product during the discharge process, as well as the high electrical conductivity, results in the superior cycling and rate performances of Co-bNCNTs hollow microspheres applied as the cathode material for Li-air batteries.

4. Conclusions Hierarchical hollow microspheres grafted with metallic Co-embedded bamboo-like NCNTs as an oxygen electrode for Li-O2 batteries were prepared via a facile two-step process. The Co3O4-MgO hollow microspheres were prepared by facile one-pot spray pyrolysis, followed by growth of bamboo-like NCNT bundles on the internal and external surfaces of the microspheres by decomposition of dicyandiamide during post-treatment. The unique structure and introduction of metallic Co and N doping in the composites could offer not only ample active sites for the OER and ORR, but also sufficient space for storing the Li 2O2 product. Thus, when applied as an oxygen electrode for Li–O2 batteries, Co-b-NCNTs hollow microsphere exhibit better electrochemical performances compared with those of Co-NCNTs and Co-MgO hollow microspheres. This new concept could be efficiently used in a wide range of electrochemical applications.

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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel.: +82-2-928-3584. Fax: +82-2-3290-3268. (Yun Chan Kang)

ACKNOWLEDGEMENTS This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. 2017R1A2B2008592).

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Figure Caption Fig. 1. (a) Formation mechanism of Co-b-NCNTs hollow microsphere and (b) schematic of discharge and charge mechanism for Li2O2 deposition and dissociation. Fig. 2. Morphologies of Co 3O4-MgO hollow microspheres: (a) SEM image, (b, c) TEM images, (d) HR-TEM image (e) SAED pattern, and (f) elemental mapping images. Fig. 3. Morphologies of Co-b-NCNTs microspheres: (a) SEM images, (b, c) TEM images, (d) HR-TEM image (e) SAED pattern, and (f) elemental mapping images. Fig. 4. (a) XPS survey scan of Co-b-NCNTs microspheres and high resolution XPS spectra of (b) C 1s, (c) N 1s, and (d) Co 2p of Co-b-NCNTs microspheres. Fig. 5. Morphologies of Co-NCNTs: (a) SEM image, (b, c) TEM images, (d) HR-TEM image (e) SAED pattern, and (f) elemental mapping images. Fig. 6. (a) ORR and (b) OER catalytic performances of Co-b-NCNTs hollow microspheres in comparison with Co-NCNTs, Co-MgO hollow microspheres, and commercial Pt-C and RuO2. Fig. 7. (a) Initial charge-discharge profiles of the Li-O2 cell with Co-b-NCNTs hollow microspheres, Co-NCNTs and Co-MgO hollow microspheres as the catalyst at a current density of 200 mA g -1, (b) cycling performances of the three electrodes with a restriction of the capacity to (b) 500 and (c) 1000 mA h g-1 and (d) terminal voltage profiles of the three electrodes with a limited capacity of 500 mA h g -1. Fig. 8. Morphologies of (a) Co-b-NCNTs hollow microspheres, (b) Co-NCNTs and (c) CoMgO hollow microspheres obtained after the 30th fully charged state at a cutoff capacity of 500 mA h g-1. Fig. 9. (a) Initial cycle profiles and (b-d) morphologies of Co-b-NCNTs hollow microspheres after first discharge and recharge at various current density: (b) 500, (c) 1000, and (d) 2000 mA g-1.

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Fig. 1. (a) Formation mechanism of Co-b-NCNTs hollow microsphere and (b) schematic of discharge and charge mechanism for Li2O2 deposition and dissociation.

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Fig. 2. Morphologies of Co 3O4-MgO hollow microspheres: (a) SEM image, (b, c) TEM images, (d) HR-TEM image (e) SAED pattern, and (f) elemental mapping images. 27

Fig. 3. Morphologies of Co-b-NCNTs microspheres: (a) SEM images, (b, c) TEM images, (d) HR-TEM image (e) SAED pattern, and (f) elemental mapping images. 28

Fig. 4. (a) XPS survey scan of Co-b-NCNTs microspheres and high resolution XPS spectra of (b) C 1s, (c) N 1s, and (d) Co 2p of Co-b-NCNTs microspheres.

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Fig. 5. Morphologies of Co-NCNTs: (a) SEM image, (b, c) TEM images, (d) HR-TEM image (e) SAED pattern, and (f) elemental mapping images. 30

Fig. 6. (a) ORR and (b) OER catalytic performances of Co-b-NCNTs hollow microspheres in comparison with Co-NCNTs, Co-MgO hollow microspheres, and commercial Pt-C and RuO2.

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Fig. 7. (a) Initial charge-discharge profiles of the Li-O2 cell with Co-b-NCNTs hollow microspheres, Co-NCNTs and Co-MgO hollow microspheres as the catalyst at a current density of 200 mA g -1, (b) cycling performances of the three electrodes with a restriction of the capacity to (b) 500 and (c) 1000 mA h g-1 and (d) terminal voltage profiles of the three electrodes with a limited capacity of 500 mA h g -1.

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Fig. 8. Morphologies of (a) Co-b-NCNTs hollow microspheres, (b) Co-NCNTs and (c) CoMgO hollow microspheres obtained after the 30th fully charged state at a cutoff capacity of 500 mA h g-1.

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Fig. 9. (a) Initial cycle profiles and (b-d) morphologies of Co-b-NCNTs hollow microspheres after first discharge and recharge at various current density: (b) 500, (c) 1000, and (d) 2000 mA g-1. 34

Graphitic abstract

Hierarchical Hollow Microspheres Grafted with Co NanoparticleEmbedded Bamboo-like N-Doped Carbon Nanotube Bundles as Ultrahigh Rate and Long-Life Cathodes for Rechargeable Lithium-Oxygen Batteries Jung Hyun Kim +, Seung-Keun Park +, Yeon Jong Oh, Yun Chan Kang * + These authors contributed equally to this work

Hierarchical hollow microspheres grafted with Co nanoparticle-embedded bamboo-like Ndoped CNT bundles were first introduced as efficient cathodes for Li-O2 batteries. The synergetic effect of the presence of the N-doped CNTs and the hierarchical structure enabling uniform deposition of the Li2O2 product were responsible for the superior performances of Co-b-NCNTs hollow microspheres as the cathode material for Li-O2 batteries. 35

Highlights - Co-embedded bamboo-like N-doped carbon nanotube bundles are studied as cathode for Liair batteries. - Hierarchical structure provides active sites for the OER and ORR and sufficient space for storing Li2 O2. - Hierarchical composite exhibit high initial round-trip efficiency, long-term cycling and ultrahigh rate performance.

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