Perovskite Sr0.9Y0.1CoO3âÎ´ Nanorods Modified with CoO

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Perovskite Sr0.9Y0.1CoO3-# nanorods modified with CoO nanoparticles as a bifunctional catalyst for rechargeable Li-O2 batteries Jie Wang, Xiaopeng Cheng, Zhaolong Li, Meng Xu, Yao Lu, Shengming Liu, Yuefei Zhang, and Chunwen Sun ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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ACS Applied Energy Materials

Perovskite

Sr0.9Y0.1CoO3-δδ

Nanorods

Modified

with

CoO

Nanoparticles as a Bifunctional Catalyst for Rechargeable Li-O2 Batteries Jie Wang1,2, Xiaopeng Cheng3, Zhaolong Li4, Meng Xu5, Yao Lu1,2, Shengming Liu1,5, Yuefei Zhang,3 Chunwen Sun1,2,6* 1

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P.R. China 2 School of Nanoscience and Technology, Unniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China 3 Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, P.R. China 4 Department of Radiation Science and Technology, Delft University of Technology, Mekelweg15, 2629JB Delft, The Netherlands 5 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China 6 Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning 530004, China *Corresponding author, Email: [email protected] (C. Sun)

Abstract For practical application of lithium-oxygen batteries, one of the challenges is the development of efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in cathode. In this work, perovskite Sr0.9Y0.1CoO3-δ nanorods are synthesized by an electrospinning method. The performance of the Li-O2 cell with Sr0.9Y0.1CoO3-δ catalysts is better than that of the cell only with Super-P. Furthermore, modification of CoO nanoparticles on the cathode can provide an obviously improved electrochemical performance with a reduced voltage gap (~80-140 mV), which is ascribed to the superior catalytic activity of CoO nanoparticles toward OER. All these results demonstrate that the perovskite Sr0.9Y0.1CoO3-δ is an efficient bifunctional electrocatalyst for lithium-oxygen batteries, and the incorporation of CoO nanoparticles is an effective approach for improving the cathode performance as well.

Keywords: Li-O2

batteries,

Sr0.9Y0.1CoO3-δ,

electrospinning, electrochemical

1

ACS Paragon Plus Environment

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properties, cycling stability.

Introduction Recently, replacing the conventional fossil energy to the clean and sustainable energy sources, such as wind, solar, ocean, biomass, geothermal and nuclear energy, is of greater importance owing to the growth of energy demand, the gradual depletion of non-renewable resources and ecological concerns. However, such alternative energy sources (solar, wind, etc.) suffer from the intermittent characteristic, requiring advanced electrochemical devices to store and deliver the electric energy on demand.1-4 Lithium ion batteries have been developed and employed successfully in portable electric devices, transportation, local grid energy storage and aerospace because of long cycle-life, no memory effect, and environmental benignity. However, the energy density of the present lithium ion batteries cannot satisfy the ever-increasing energy requirements.5-8 In recent years, lithium-oxygen (Li-O2) batteries have attracted much attention as a promising candidate because of its simple reaction chemistry (2Li++O2+2e-→Li2O2, Eo = 2.96 V vs. Li+/Li) that gives a higher theoretical energy density (11700 Wh kg-1), which are expected to be potential to replace the state-of-the-art lithium ion batteries.9-14 However, Li-O2 batteries suffer from the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics, low energy efficiency and poor cyclability.15,16 The formation of discharge products Li2O2 with insulating characteristic endows the reaction processes with

high

polarization,

poor

rate

capability

and

reversibility.17,18

The

formation/decomposition processes of Li2O2 have been viewed as a major obstacle for improving the electrochemical performance of Li-O2 batteries, in terms of rate-capability and cycling stability.12 Thus, development of highly efficient bifunctional catalysts both for ORR and OER is highly desired for practical application of Li-O2 batteries. So far, the reported catalysts include noble metals, metal oxides, alloys, carbon-based materials, and others (metal nitrides, metal phosphides, etc.).19-22 For example, Shao-Horn group reported PtAu/C material with good bifunctional catalytic 2


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activity. The surface Au and Pt nanoparticles were catalytically effective towards the ORR and OER processes, respectively.23 Lin et al. prepared the binder-free Pt-Gd polycrystalline thin film via a pulsed laser deposition (PLD) technique, which showed favorable performance with higher round-trip efficiency, resulting from the higher catalytic activity of the Pt-Gd alloy composite.24 Hierarchically porous RuO2 hollow spheres, reported by Li et al., showed an excellent electrochemical performance: an average charge voltage as low as 3.59 V, fully discharged/charged for 100 cycles.25 Ma et al. synthesized noble metal (Ru and Pd)-catalyzed carbon nanotube fabrics via magnetron sputtering method, which exhibit excellent electrochemical performance with high round-trip efficiency and good cycle life.26 Wu et al. prepared carbon-free CoO mesoporous nanowire array cathode via a hydrothermal method. The synthesized cathode delivers a specific capacity of 4800 mAh g-1CoO and long-term stability for 50 cycles.27 Despite the mentioned catalysts above, perovskite oxides, with a general formula of ABO3, have been investigated as promising catalysts in many energy conversion and storage systems with excellent specific catalytic activity towards ORR/OER owing to their defective structures and excellent oxygen mobility.19,28,29

Typically,

La0.6Sr0.4Co0.8Mn0.2O3,34

La0.5Sr0.5CoO3-δ,29-31 Ba0.9Co0.7Fe0.2Nb0.1O3-δ,35

La0.6Sr0.4Co0.2Fe0.8O3,32,33 La0.75Sr0.25MnO3,36

La0.65X0.35MnO3 (X=Sr, Ba, Pb),37 LaNiO3,38-40 Sr0.95Ce0.05Co3-δ,41 have been investigated as the cathode catalysts for Li-O2 batteries. Although the introduction of perovskite oxides can directly lead to the performance enhancement, the particles size, the defect state and microstructure of the air electrode are the crucial factors determining the electrochemical performance. Among various perovskite oxides with mixed oxygen ion and electron conductivity, SrCoO3-δ-based materials, are of great interest owing to their cubic perovskite structure with high reversible ORR/OER catalytic activity,42 which has been widely investigated in solid oxide fuel cells and water splitting fields.43-47 However, their structural instability at high temperatures implies the necessity of stabilizing the crystal structure by chemically compensating the lattice oxygen loss via donor-doping either at A site or at B site.48 Sr0.95Ce0.05CoO3-δ particles loaded with copper 3



nanoparticles was successfully synthesized and shows a stable and improved bifunctional catalytic activity for lithium-air batteries.41 Moreover, one dimensional (1D) nanofibers with high aspect ratio and porosity usually exhibit superior performance toward the ORR/OER reaction since they can maximize the catalytic sites and are beneficial for the diffusion of electrons and reactants. Herein, we report perovskite Sr0.9Y0.1CoO3-δ oxide as an efficient bifunctional catalyst for rechargeable lithium-oxygen batteries. The Sr0.9Y0.1CoO3-δ nanorods were synthesized by a facile electrospinning method and the subsequent calcination. The Li-O2 cell with Sr0.9Y0.1CoO3-δ nanorods catalyst exhibits a higher specific discharge capacity of ~2000 mAh g-1carbon compared to the cell only with SP cathode (~1200 mAh g-1carbon) at 50 mA g-1carbon, and a long-term stability over 140 cycles at a current density of 50 mA g-1carbon with a limited capacity of 500 mAh g-1carbon. Moreover, considering CoO has high catalytic activity towards OER and excellent cycling stability owing to the favorable adsorption configuration of LiO2 on CoO surface,49 the surface of cathode was further modified by CoO nanoparticles via magnetron sputtering, which can significantly lead to the reduction of voltage gap about 80-140 mV.

Experimental Synthesis of the Sr0.9Y0.1CoO3-δ (SYC-NRs) nanorods The SYC NRs were synthesized by an electrospinning method and the subsequent calcination. All the regents were used as received with analytical grade. In a typical synthesis, stoichiometric amounts of Sr(NO3)2, Y(NO)3∙6H2O and Co(NO3)2∙6H2O were dissolved in N,N-dimethylformamide (DMF). Then, polyvinylpyrrolidone (PVP) was added to the above solution. The mixture was stirred overnight for the complete dissolution of PVP, then the as-prepared solution was transferred into a 10 ml plastic syringe with a 22-gauge stainless steel spinneret. A voltage of 20 kV was applied in the experiment, and the distance between the spinneret tip and the aluminum foil collector was fixed at 20 cm. After electrospinning, the collected electrospun nanofibers were calcined at 900 oC for 3 h at a heating rate of 3 oC min-1 to obtain 4


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SYC-NRs with high purity. Materials characterizations XRD data were collected on the X-ray diffraction (XRD, PANalytical X’Pert3 Powder), equipped with Cu Kα radiation operating at a tube voltage of 40 kV and a current of 40 mA, over the scanning range of 10o-80o with an interval of 0.02o. The morphologies of the synthesized materials were characterized by field-emission scanning electron microscope (FESEM, SU8020) and transmission electron microscope (TEM, JEM-2100F). The HAADF-STEM (Oxford X-maxN80T) equipped with an EDX analyzer at 300 kV. X-ray photoelectron spectroscopy (XPS) was performed on a spectrometer with Mg Kα radiation (ESCALAB 250, Thermofisher Co.). The data were fitted by using XPSPEAK software. Operando XRD analysis was performed on a PANalytical X’Pert Pro PW3040/60 diffractometer, with Cu Kα radiation operating at 45 kV and 40 mA, in a 2θ range of 30-70°. The Brunauer-Emmett-Teller (BET) surface area was measured with Automated Surface Area analyzer (QUADRASORB SI-MP, Quantachrome, USA). Preparation of SYC-NRs/SP and CoO-coated SYC-NRs/SP cathodes The SYC-NRs/Super-P carbon black (SP, Alfa Aser) electrodes were prepared by casting a homogeneous slurry, composed of 60 wt.% Super-P carbon black, 30 wt.% SYC NRs, and 10 wt.% polyvinylidene fluoride (PVdF) in N-methyl-2-pyrrolidone (NMP), on a porous carbon paper. The prepared electrodes were dried at 120 oC for 12 h under vacuum before electrochemical tests. The mass loading of carbon on the as-fabricated electrodes was ~0.5±0.1 mg cm-2. Similarly, the pure SP electrodes were prepared without the addition of SYC NRs catalyst (90 wt.% Super-P carbon black and10 wt.% PVdF). For preparation of CoO-coated SYC-NRs/SP cathode, CoO deposition was performed on the prepared SYC-NRs/SP electrode at room temperature by DC-magnetron sputtering technique (Kurt J. Lesker PVD75 Proline, USA) with 99.99% pure cobalt target (4 inches) and pure argon as the sputtering gas. The sputtering chamber was evacuated to ∼3×10-5 Torr prior to the introduction of Ar gas. The 5



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working pressure was controlled at 3.6×10-3 Torr in Ar ambient. The flow rate of Ar was controlled at 18.5 sccm. The DC-power and the deposition time were fixed at 25 W and 30 min, respectively. During sputtering, the substrate holder was spun at 20 rpm. Electrochemical measurements The electrochemical properties of lithium-oxygen batteries were examined by galvanostatic cycling with a standard 2032-type stainless-steel coin cells with a few holes on one side. The cells were consisted of SYC-NRs/SP, lithium metal, and porous glass fiber as working electrodes, counter electrode, and separator, respectively. The electrolyte used was 1 M lithium bis-(trifluoromethanesulfonyl)-imide (LiTFSI) in tetraethlene glycol dimethyl ether (TEGDME). All the batteries were assembled in an Argon filled glove box, and were transferred into a glass container filled with pure oxygen

for

electrochemical

performances

investigation.

Galvanostatic

discharge/charge tests were carried out in the voltage range of 2.0-4.5 V vs. Li+/Li with a battery testing system (LAND CT2011A tester, Wuhan, China) at room temperature. The specific capacities of the electrode were calculated on the basis of the mass of SP. Cyclic voltammetry (CV) measurement was performed on a CHI 604E electrochemical workstation at a scan rate of 1 mV s-1.

Results and discussion The SYC-NRs were synthesized by a facile electrospinning process. CoO nanoparticles were further deposited on the electrode by magnetron sputtering method. Figure 1a shows the unit cell of the ABO3 perovskite structure. Figure 1b shows schematic illustration of the synthesis of SYC-NRs by electrospinning and the preparation process of CoO nanoparticles coated electrode. Figure 1c presents XRD patterns of the synthesized SYC-NRs at different temperatures. The XRD results reveal that the synthesized sample without Y doping displays a distorted 2H BaNiO3-type structure. Doping Y at Sr site can effectively stabilize the crystal structure into perovskite phase at room temperature. With the calcination temperature 6


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increasing up to 900 oC, all the diffraction peaks can be assigned to a well-crystallized cubic perovskite oxides, except the peaks centered around 42o and 48o, which are related to a tetragonal superlattice structure (P4/mmm) with lattice parameters of at=bt ≈a0, ct=2a0, where a0 is the lattice parameter of the cubic unit cell.50,51 The stronger peak intensity indicates a high crystalline, which is also confirmed by high-resolution transmission electron microscopy (HRTEM) (Figure 1g, Figure S1b). The d-spacing of lattice fringes is 0.266 (0.268) and 0.379 nm, corresponding to (110) and (100) planes of the perovskite structure, respectively. The morphology of the as-prepared polymer fibers precursor displays an average diameter of ~300 nm (Figure 1d). After calcination at 900 oC for 3h, the fibers break and shrink into nanorods with an average diameter of ~200 nm. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and the corresponding element mapping images suggest the homogenous distribution of Sr, Y, Co, O elements in the SYC-NRs sample. Besides, the energy dispersive X-ray (EDX) spectrum of SYC-NRs gives an atomic ratio of 0.89:0.11:1 for Sr:Y:Co, in good agreement with the expected stoichiometry in the synthesized perovskite phase (Figure 1i). After depositing cobalt-based species on the prepared SYC-NR/SP electrode by a DC-magnetron sputtering technique, the SYC-NRs surface was coated with a layer composed of tiny crystalline nanoparticles, with a thickness of 1-5 nm, as shown in Figure 1j and Figure S1. The d-spacings of 0.246 and 0.207 nm match well with the (111) and (200) planes of CoO phase, respectively. The porous structure and pore size are investigated by nitrogen adsorption-desorption measurements, as shown in Figure S2 (Supporting Information). The BET specific surface areas of the synthesized SYC-NRs and commercial SP sample are 3.54 m2 g-1 and 74.49 m2 g-1, respectively.

7



Figure 1. (a) Schematic of ABO3-perovskite structure; (b) Schematic illustration of the synthesis of SYC-NRs by electrospinning and the preparation process of CoO-coated electrode; (c) XRD patterns of the synthesized SYC-NRs samples; (d) SEM image of the as-spun polymer nanofibers precursor; (e) SEM image of the synthesized SYC-NRs at 900 oC without grinding; (f) SEM image of the synthesized SYC-NRs at 900 oC after grinding in the mortar; (g) TEM image of the synthesized SYC-NRs; (h) HRTEM, HAADF-STEM and the corresponding EDX element mapping images; (i) EDX spectrum of the synthesized SYC-NRs sample; (j) HRTEM of the synthesized CoO-coated SYC-NRs sample. Insets in (d), (e) and (f) are the corresponding magnified SEM images; inset in (g) is the corresponding HRTEM image.

X-ray photoelectron spectroscopy (XPS) was used to examine the composition of the sample and the valence state of the Co ions. As shown in Figure 2, the two core-level signals of Co in both electrodes located at ~780 and 796 eV correspond to 8


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Co2p3/2 and Co2p1/2, respectively. For SYC-NRs/SP electrode, the peaks can be deconvoluted into two peaks, assigned to Co3+ and Co4+, respectively. For the electrode treated by magnetron sputtering, the deconvoluted peaks at 780.9 and 796.2 eV belong to Co2+2p3/2 and Co2+2p1/2, while those peaks at 779.7 and 795.1 eV as well as 782.3 and 797.7 eV are related to Co3+2p3/2/Co3+2p1/2 and Co4+2p3/2/Co4+2p1/2, respectively.52-54 The area ratio of Co3+ 2p3/2 to Co4+ 2p3/2 peaks is 0.608, as shown in Figure 2a. The calculated surface Co average valence and the oxygen nonstoichiometry are 3.62 and 2.86, respectively. Based on the XPS analysis results, combined with the mass ratio and the density of SYC-NRs and SP component, the mass ratio of CoO on the SYC-NRs surface can be calculated around 2.5 wt.%. The HRTEM and XPS results indicate successful depositing CoO nanoparticles on the surface of SYC-NRs.

Figure 2. XPS spectra of Co 2p of the synthesized SYC-NRs/SP electrode (a) and CoO-coated SYC-NRs/SP electrode (b).

To investigate the ORR/OER catalytic activities of the prepared SYC-NRs/SP and CoO-coated SYC-NRs/SP electrodes, CV measurement were further performed in the potential range of 2.0-4.5 V (vs. Li+/Li), as shown in Figure S3. It can be seen that anodic and cathodic currents are increased and the cell with the SYC-NRs/SP cathode show the highest ORR onset potential, indicating better ORR/OER activity. Although further CoO-coating results in the slightly decreased ORR onset potential, the highest anodic current density indicates high OER activity, which is favorable for the Li2O2 decomposition. The excellent bifunctional electrocatalytic activities of SYC-NRs/SP 9



and CoO-coated SYC-NRs/SP electrodes enable them as efficient catalyst for Li-O2 cells. The stable operation of a nonaqueous Li-O2 battery relies on the reversible formation/decomposition of insoluble discharge product of Li2O2 on the cathode during discharge/charge processes. We fabricated Li-O2 batteries with a nonaqueous electrolyte. The full discharge and charge performance was examined at a current density of 50 mA g-1carbon, as shown in Figure 3. The commercial SP with good electronic conductivity was used to improve conductivity and capability of Li2O2 storage, while the addition of SYC-NRs catalyst helps to promote the formation and decomposition of Li2O2 during cycling. At a current density of 50 mA g-1carbon, the cell with SYC-NRs catalyst shows much improved cycling performance in terms of cycling stability, specific capacity and Coulombic efficiency (~100%). The cell with SYC-NRs/SP electrode exhibits a higher initial discharge capacity (~1800 mAh g-1carbon) than that of the cell with pure SP electrode (~1200 mAh g-1carbon), with a capacity increase rate of ~50%. After 5 cycles, the discharge capacities of the cells with SYC-NRs/SP electrode and SP electrode are ~1780 and 990 mAh g-1carbon, respectively. The capacity retentions of the cells with the SYC NRs -SP electrode and the SP electrode are 98.9 and 82.5%, respectively. The unique characteristics of SYC-NRs for ORR/OER favor the reversible formation/decomposition processes of Li2O2, effectively avoiding the loss of active sites during cycling.

Figure 3. (a,b) Cyclic performance and the Coulombic efficiency of the pure SP 10


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electrode, SYC-NRs/SP electrode, and CoO-coated SYC-NRs/SP electrode at a current density of 50 mA g-1carbon.

Such enhanced performance can also be evidenced by comparing the initial discharge/charge curves (Figure 4a) with a lower voltage gap for SYC-NRs/SP electrode, as well as comparing with that of the cell with pure SP electrode. It is reported that compounds of cobalt perform better toward OER.27,54-56 Therefore, depositing CoO nanoparticles was applied for further improving the electrochemical performance. The cell with CoO-deposited SYC-NRs/SP electrode shows an even higher discharge capacity over 3000 mAh g-1carbon while the charging voltage of the cell is lower than that of the cell with SYC-NRs/SP electrode (Figure 4a), demonstrating the critical role of CoO nanoparticles on the surface in catalyzing OER kinetics. It is worthy to note the increase of the specific capacity upon cycling, especially for the cell with CoO-coated SYC-NRs/SP electrode, is attributed to a certain activation process, such as the establishment of the three-phase boundary. It is noted that the Coulombic efficiency of the cell tends to be over 100% with cycling, which may be related to the electrolyte decomposition above 4.0 V versus Li+/Li.

Figure 4. (a) Comparison of the initial discharge/charge curves of the Li-O2 batteries 11



with the pure SP, SYC-NRs/SP and CoO-coated SYC-NRs/SP electrodes at a current density of 50 mA g-1carbon. (b~d) The initial discharge/charge curves of Li-O2 batteries with various catalysts at various current densities: (b) pure SP, (c) SYC-NRs/SP and (d) CoO-coated SYC-NRs/SP electrodes.

Figure 4b-d shows the rate capability of the synthesized three electrodes at current densities of 50, 100, 200 and 400 mA g-1carbon, respectively. With increasing the current densities, all the discharge/charge voltage gaps of the cells increase and the discharge capacity decrease. Compared to the cell with pure SP electrode, the cells with SYC-NRs/SP and CoO-coated SYC-NRs/SP electrodes deliver higher specific capacities at each current density. Even though the cell with SYC-NRs/SP electrode shows a similar capacity to that of the cell with the CoO-coated SYC-NRs/SP electrode, the relatively higher Coulombic efficiency of the cell with CoO-coated SYCNRs-SP electrode demonstrates a better reversibility during cycling, especially at higher current densities, resulted from the much better catalytic activity of CoO toward OER. Figure 5 shows the cycling performance of the cell at a current density of 50 mA g-1carbon with a limited capacity of 500 mAh g-1carbon. The Li-O2 batteries with SYC-NRs/SP and CoO-coated SYC-NRs/SP electrode can cycle for more than 100 cycles, while the Li-O2 batteries with SP electrode can only sustain 25 cycles, implying the effective decomposition of the discharge products with SYC-NRs and CoO addition. For the cell with pure SP electrode, the short cycle life accompanying with lower Coulombic efficiency results from the poor OER catalytic behavior, accompanying the active sites loss during cycling. For SYC-NRs/SP and CoO-coated SYC-NRs/SP electrodes, the discharge and charge capacities of the cells retain stable values, and the discharge terminal voltage is higher than 2.5 V, and the charge terminal potential is lower than 4.4 V after 100 cycles. In particular, the cell with CoO-decorated electrode displays more stable cycling performance. The superior cycling performance should be attributed to the higher OER activity of the SYC-NRs and CoO catalysts in effectively catalyzing the decomposition of the discharge 12


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product Li2O2 during charging process. Besides, the alleviated side reactions due to the modification of CoO may lead to the enhanced cycling performance.49 It is worthy to note that even though the Li-O2 batteries with CoO-coated SYC-NRs/SP electrode exhibit a reduced voltage gap compared to that of SYC-NRs/SP electrode during the first 20 cycles, it degrades to the same level in the following cycles. Besides, compared with the cell with SYC-NRs/SP electrode, the discharge terminal voltage of the cell with CoO-coated SYC-NRs/SP electrode shows a little bit fast decrease during the first 30 cycles, implying that the deposited CoO nanoparticles on the surface of cathode may have a negative effect on the ORR process during cycling.

Figure 5. Galvanostatic discharge/charge curves of the Li-O2 batteries with pure SP electrode (a), SYC-NRs/SP electrode (b), and CoO-coated SYC-NRs/SP (c) electrode at a current density of 50 mA g-1carbon with a limited capacity of 500 mAh g-1carbon; (d) discharge capacity and discharge/charge terminal voltages versus cycle numbers at a current density of 50 mA g-1carbon with a limited capacity of 500 mAh g-1carbon.

It is commonly known that the depth of discharge process can greatly affect the cycle stability for all types of lithium batteries. To further verify the effect of CoO coating on the electrochemical performance, both of the cells were tested in the 13



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voltage windows of 2.0-4.5 V under a galvanostatic mode with a limited capacity of 1000 mAh g-1carbon at a current density of 50 and 100 mA g-1carbon, respectively. As shown in Figure 6a-c, at each current density, the cell with CoO-coated SYC-NRs/SP electrode performs more stable than the cell with SYC-NRs/SP electrode, suggesting the improved durability of the cell with CoO coating. Specifically, the Li-O2 batteries with CoO-coated SYC-NRs/SP can cycle for more than 55 and 30 cycles with much lower charge voltage, whereas the Li-O2 batteries with SYC-NRs/SP electrode display a capacity fading starting from 37 and 20 cycles, at a current density of 50 and 100 mA g-1carbon, respectively. As the current density increases up to 200 mA g-1carbon, the cell with CoO-coated SYC-NRs/SP electrode shows almost two times long cycle life than the cell with SYC-NRs/SP electrode with a limited capacity of 500 mA g-1carbon (Figure 6d). Such observations indicate that the deposited CoO nanoparticles catalyst is beneficial for the decomposition of Li2O2 during long-term cycling, originating from high activity of CoO in catalyzing OER kinetics. Although the cell with CoO-coated SYC-NRs/SP electrode displays a more stable cycling performance, it is noted that the discharge voltage value of the cell presents a bigger degradation rate during the initial cycles, which is in consistent with the results shown in Figure 5. This is presumably resulted from the insufficient ORR property of the electrode with CoO coating. Therefore, it is still need to further optimize the amount of CoO coating. Table S1 shows the electrochemical performance comparison with the reported works. Considering the depth of the discharge process using in our test (limited capacity of 500 mAh g-1carbon: ~25%; limited capacity of 1000 mAh g-1carbon: ~50%), the cycling stability of the cells with the present CoO-coated SYC-NRs/SP electrodes is superior to those results reported in Table S1. The electrochemical performances of the cell with CoO-coated SYC-NRs/SP electrode are also comparable to the cells with La0.6Sr0.4CoO3

electrode,57,58

La0.5Sr0.5CoO3-δ

nanotube

electrode,59

Ba0.9Co0.7Fe0.2Nb0.1O3-δ electrode,35,60 La0.65X0.35MnO3 (X=Sr, Ba, Pb) electrode,37 La0.6Sr0.4Co0.9Mn0.1O3 [email protected]

electrode,61

La0.8Sr0.2Mn0.6Ni0.4O3

electrode,63

N-doped

LaNiO3

electrode,62 electrode,64

La0.8Sr0.2Co0.8Fe0.2O3 electrode,65 ZnCo2O4 electrode,66 and graphene/meso-LaSrMnO 14


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sandwich-like nanosheets,67 etc.

Figure 6. Galvanostatic discharge/charge curves of the Li-O2 batteries with SYC-NRs/SP and CoO-coated SYC-NRs/SP electrodes at a current density of (a) 50 and (b) 100 mA g-1carbon with a limited capacity of 1000 mAh g-1carbon; (c) discharge capacity and discharge/charge terminal voltages versus cycle numbers at a current densities of 50 and 100 mA g-1carbon with a limited capacity of 1000 mA g-1carbon; (d) discharge capacity against cycle numbers at a current densities of 200 mA g-1carbon with a limited capacity of 500 mA g-1carbon.

Operando XRD analysis on the CoO-coated SYC-NRs/SP electrode was carried out to gain insight into the products formed during discharge/charge cycle with a limited capacity of 2 mAh at a current density of 50 mA g-1carbon. As shown in Figure 7, the peaks at 32.6, 40.3, 45.7 and 58.3o correspond to the (110), (111), (200) and (211) planes of the perovskite phase (SYC-NRs) respectively, while the peak at 54.4o is 15



related to the carbon paper. As the discharge process goes on, the peak at 35.1o assigned to the (101) lattice plane of Li2O2 tends to become more obvious, indicating the gradual formation of the discharge product Li2O2. Such broadened peak with relatively low intensity suggests the poor crystallinity of the formed Li2O2 during operando XRD test. During the charge process, the peak of Li2O2 gradually disappears, indicating the decomposition of Li2O2. At the end of charge process, the peak of Li2O2 disappears. This result demonstrates that the CoO-coated SYC-NRs can catalyze effectively the complete decomposition of the discharged products. Moreover, the LiOH formation/decomposition processes are also observed resulted from the reaction between Li2O2 and moisture during cycling.

Figure 7. Operando XRD patterns of the CoO-coated SYC-NRs/SP electrode during discharge/charge processes with a limited capacity of 2 mAh (50 mAg-1carbon).

The enhanced performances of the CoO-coated SYC-NRs and SYC-NRs/SP electrodes can be attributed to the following aspects: (1) The surface generated . electron holes ( ) in Sr0.9Y0.1CoO3-δ can facilitate charge transfer; (2) The

increased oxygen chemisorption and lattice oxygen mobility, as well as the accelerated surface O2 dissociation process due to the oxygen vacancies in the SYC can enlarge O2 transfer pathway and decrease accumulation of intermediate product LiO2, resulting in the formation of Li2O2;19 (3) The generated LiO2 during charge 16


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process can interact with oxygen vacancies and thus increase the OER activity;68,69 (4) The deposited CoO nanoparticles with high OER catalytic activity not only favors the Li2O2 decomposition process, but also alleviates the side reactions.49 All these results demonstrate that the perovskite-type SYC-NRs material is a promising bifunctional catalyst for rechargeable Li-O2 batteries, and much higher OER activity can be realized by depositing CoO nanoparticle on the surface of cathode. Besides, the CoO-depositing technique via magnetron sputtering is simple and effective, which can be extended to prepare other electrodes for metal-air batteries.

Conclusions In summary, perovskite-type SYC-NRs catalyst was successfully synthesized by a simple electrospinning method. As a catalyst for Li-O2 batteries, the synthesized SYC-NRs/SP electrode displays an improved electrochemical performance, in term of high specific capacity, good cycle stability and rate capability. The enhanced batteries performance should be attributed to the highly catalytic activity of SYC-NRs for ORR/OER. Further modification of CoO nanoparticles on the electrode surface by a simple magnetron sputtering technique effectively decreases the discharge/charge voltage gap and leads to a longer cycle life. In particular, the incorporation of CoO nanoparticles on the surface of cathode shows enhanced OER catalytic behavior and improved cycling performances. These results demonstrate that SYC-NRs material is a promising bifunctional catalyst for Li-O2 batteries. The deposition of CoO nanoparticles via magnetron sputtering provides a simple and efficient strategy for designing the cathode with high performance.

Acknowledgements The authors acknowledge the financial support of the National Science Foundation of China (Nos. 51672029, 51372271 and 51474009) and the National Key R & D Project from Ministry of Science and Technology, China (2016YFA0202702). This work was also supported by the Thousands Talents Program for the pioneer researcher and his innovation team in China. 17



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Nitrogen adsorption/desorption isotherms, TEM and HRTEM of the CoO-coated SYC-NRs material and Electrochemical performance comparison of Li-O2 batteries with different cathode catalysts.

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Table of Contents (TOC) Perovskite Sr0.9Y0.1CoO3-δ nanorods are synthesized by an electrospinning technique followed by heat treatment. CoO nanoparticles are deposited on the electrode surface via magnetron sputtering technique. As a catalyst for Li-O2 batteries, it provides an obviously improved electrochemical performance with a reduced voltage gap (~80-140 mV) and excellent cyclability.

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Perovskite Sr0.9Y0.1CoO3âÎ´ Nanorods Modified with CoO

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Perovskite Sr0.9Y0.1CoO3âÎ´ Nanorods Modified with CoO