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Facile Synthesis of Hierarchical Porous Three-dimensional Free-standing MnCoO Cathodes for Long-life Li-O Batteries 2

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Haitao Wu, Wang Sun, Yan Wang, Fang Wang, Junfei Liu, Xinyang Yue, Zhenhua Wang, Jinshuo Qiao, David W. Rooney, and Kening Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16090 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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ACS Applied Materials & Interfaces

Facile Synthesis of Hierarchical Porous Threedimensional Free-standing MnCo2O4 Cathodes for Long-life Li-O2 Batteries Haitao Wu,† Wang Sun,*,†,‡ Yan Wang,† Fang Wang,† Junfei Liu,† Xinyang Yue,† Zhenhua Wang,†,‡ Jinshuo Qiao,† David W. Rooney§ and Kening Sun*,†,‡ †

Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemistry

and Chemical Engineering, Beijing Institute of Technology, Beijing, 100081, People's Republic of China. ‡

Collaborative Innovation Center of Electric Vehicles in Beijing, No.5 Zhongguancun South

Avenue, Haidian District, Beijing, 100081, People's Republic of China §

School of Chemistry and Chemical Engineering, Queen’s University, Belfast, Northern Ireland

BT9 5AG, United Kingdom KEYWORDS: lithium-oxygen batteries, free-standing catalysts, MnCo2O4 nanowires, binderfree, carbon-free

ABSTRACT: Hierarchical porous three-dimensional MnCo2O4 nanowire bundles were obtained by a universal and low-cost hydrothermal method, which subsequently as a carbon-free and binder-free cathode for Li-O2 cell applications. This system showed a large discharge capacity up to 12919 mAh g-1 at 0.1 mA cm-2 as well as an outstanding rate capability. Under a constrained

ACS Paragon Plus Environment

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specific capacity of 500 and 1000 mAh g-1, the Li-O2 batteries could be successfully operated for over 300 and 144 cycles, respectively. Moreover, the charge voltage was markedly decreased to about 3.5 V. The excellent electrochemical performance is proposed to be related to the conductivity

enhancements

resulting

from

the

hierarchical

interconnected

mesoporous/macroporous web-like structure of the hybrid MnCo2O4 cathode, which facilitated electron and mass transport. More importantly, after two months of cycling, the microstructure of the cathode was maintained and a recyclability of over 200 cycles of the reassembled Li-O2 cells was achieved. The impacts of the level of electrolyte and corrosion of the lithium anode during long-term cycling on the electrochemical property of Li-O2 cells have been explored. Furthermore, the nucleation process of the discharge product morphology has been investigated.

1. Introduction Due to the urge trend of the current social and industrial towards extensive range electrical transportation and large-scale energy storage systems, great efforts have been afforded to the research and evolution of high energy density lithium-ion batteries (LIBs), which are currently advanced and widely commercial.1-7 Nevertheless, even fully developed, the highest energy storage which LIBs can deliver, in view of the intercalation chemistry, is too low to satisfy the market like future transport.4,6,7 As an alternative, Li-O2 cells have gained a lot of attention owing to their environmental friendliness and large theoretical energy density of approximately 3500 Wh kg-1, which is 5-10 times higher than current LIBs.3,4,8,9 As early as 1996, non-aqueous Li-O2 batteries had been reported.10 However, rechargeable Li-O2 cells, particularly with a nonaqueous electrolyte, gained very little attention until 2006 when Bruce and other researchers demonstrated the reversibility of the non-aqueous Li-O2 battery.3,11-13 Although many achievements have been obtained in recent years, there are still lots of obstacles that restrict the


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practical application of Li-O2 cells, such as dendrite formation, Li anode security, the stability and volatilisation of electrolyte, and especially the sluggish kinetics and overpotentials of the O2 electrode, which result in low round-trip energy efficiency, bad rate performance and poor cyclability.4,6,14 Therefore, various electrocatalysts have been studied for Li-O2 cells to catalyse the oxygen reduction reaction (ORR, O2 + e− → O2−, O2− + Li+ → LiO2, 2LiO2 → Li2O2 + O2)4 and the oxygen evolution reaction (OER, Li2O2 → O2 + 2Li) thereby improving the electrochemical performance. Noble metals acting as catalysts have achieved good round-trip efficiency and long-life cycleability, but their high cost limits their practical use.15-17 Alternatively metal oxides, particularly mixed metal oxides like cobalt and manganese based materials are of interest as bi-functional electrocatalysts for OER and ORR owing to their cheapness, excellent catalytic activity, prominent stability, and facile synthesis.18-20 However, like many metallic oxides, poor conductivities of the MnCo2O4 oxides is a significant challenge to be overcome to further improve their catalytic activity.21 Additionally, the morphology of the catalysts and the porous structure of O2 electrodes are important for the enhanced performance of Li-O2 cells.8,9,14,22 It is well known that large surface areas can provide plenty of ORR and OER active sites and that hierarchical porous structures can offer effective space for O2 diffusion and Li2O2 deposition, resulting in enhanced performance of Li-O2 batteries. Moreover, such structures increase the triple-phase boundaries (i.e. oxygencatalyst-electrolyte) which are involved in the ORR and OER, and facilitate the formation and decomposition of discharge products Li2O2 which have poor conductivity and solubility which could otherwise lead to pore blockage and failure of the Li-O2 cell. Therefore it is generally considered that designing a three-dimensional and free-standing electrocatalyst with a mesoporous/macroporous structure will result in improved performance.8,9,14 There have been


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some reports about MnCo2O4 catalysts with different synthetic methods and structures applied to Li-O2 cells, but most of them have small surface areas and pore sizes, and the free-standing MnCo2O4 electrode with hierarchical porous structure is still under studied.21-28 Furthermore, the discharge products Li2O2 and intermediates of Li-O2 batteries are highly radical with previous studies suggesting that common carbon based materials are not stable. This is particularly the case in non-aqueous Li-O2 battery systems, which can be oxidized by the highly active O2-/O2-/O22- intermediates, especially radical O2-/O22- species to form irreversible carbonates, leading to a large overpotential during the recharge process and poor cycleability.4,13,29,30 Additionally, polymer binders with poor conductivity are widely used for the fabrication of air cathodes, but may block the O2 diffusion paths and decrease the void space for Li2O2 deposition and effectively active sites.31 More seriously, polymer binders, such as PVDF would decompose to form unwanted by-products LiF and LiOH, leading to further instability of Li-O2 batteries.32-34 Hence, it is highly desired to develop novel binder-free, carbon-free and freestanding electrode materials with hierarchical porous structures to reduce side reactions, improve the ORR and OER activities, hold more Li2O2 products and resist volume change during cycling processes. In this work, hierarchical porous three-dimensional web-like MnCo2O4 nanowire bundles (WMCO-NWB), directly grown on nickel foam, were synthesized by a high efficiency and low cost method. In terms of the electrode design, the nanowires are directly contacted with the Ni foam current collector and interconnected with each other to form a web-like structure, promoting electron transport. In addition, the mesoporous/macroporous structure of the W-MCO-NWB sample can shorten the ion transport and oxygen diffusion pathways. Moreover, in order to minimize the impacts of electrolyte decomposition and get a better understanding of the


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performance of Li-O2 cells consisted with the binder-free and carbon-free cathode, a charge voltage cut off at 4.4 V, and a DMSO-based electrolyte with a good stability in non-aqueous electrolyte was used.9,35 Finally, the W-MCO-NWB composites were directly served as freestanding cathodes in the Li-O2 cells, showing a large reversible capacity of 12919 mAh g-1 as well as a good rate performance. The cycleability was maintained for over 300 and 144 cycles when the capacity was limited to 500 and 1000 mAh g-1, respectively. More interestingly, after up to two months of cycling, the microstructure of the cathode was still retained and a cycleability of over 200 cycles of the reassembled Li-O2 cell achieved, suggesting that the O2 electrode is not the main reason for the failure of the cell. Therefore, to better understand the decay mechanism of the cell, the impacts of the level of electrolyte and corrosion of lithium anode during long time cycling on Li-O2 cells were further investigated and discussed. The nucleation process of the discharge product morphology was also investigated. 2. Experimental Section 2.1. Materials Preparation. Synthesis of web-like MnCo2O4 nanowire bundles on Ni foam. A facile hydrothermal method was used to synthesize a hierarchical porous three-dimensional weblike free-standing MnCo2O4 electrode by self-assembling nanowire bundles onto a Ni foam substrate. In a typical procedure, the Ni foam (approximately 2.0 cm × 3.5 cm) was washed with a diluted HCl solution (2.0 M) for 0.5 h to clean any surface oxide species, after that washed by distilled water and ethanol for 30 min under ultrasonication, respectively. The precursor solution was obtained by mixing 1 mmol of Mn(NO3)2•4H2O, 2 mmol of Co(NO3)2•6H2O and 3 mmol of (NH4)2SO4 in 60 ml of distilled water, and stirring for 30 min for a uniform solution. 10 mmol of urea was then gradually added into the resulting solution while stirring. After mixing for another 30 min, the precursor solution was poured into an 80 ml stainless steel autoclave with Teflon-


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lined and the pretreated nickel foam was vertically placed into the reactor, which was reacted at 120 oC for 6 h. The product was immersed in deionized water with an ultrasonic cleaning for 30 s, and then rinsed with distilled water and absolute ethanol. Lastly, the MnCo2O4 precursor was calcined at 400 oC for 2 h in air. The mass loading of the MnCo2O4 electrode was about 0.5-0.8 mg cm-2. 2.2. Materials characterization. The XRD (X-ray diffraction, Rigaku Ultima IV, Cu Ka radiation, 40 kV, 40 mA) was employed for characterization of the crystal structure of samples. The micrograph of the sample was obtained by a field emission scanning electron microscopy (FESEM, Quanta FEG 250) and the TEM (transmission electron microscopy ) images were obtained via a JEOL JEM-2001F (200 kV). The valence of elements was characterized by XPS (X-ray photoelectron spectroscopy, Physical Electronics 5400 ESCA). The pore size and specific surface area of the synthesized sample were calculated by N2 adsorption-desorption measurements (Quantachrome Instrument ASIQM0VH002-5). 2.3. Electrochemical Measurements. The electrochemical property of the lithium-oxygen battery was tested by a type CR2025 coin cell assembled in a glove box with argon-filled. And the concentrations of oxygen and moisture in glove box were both less than 0.5 ppm. Seven holes with a diameter of 1.5 mm were evenly distributed on the porous electrode side for oxygen access. Free-standing MnCo2O4 was directly used as the porous electrode while the anode used a lithium metal, and separated by a glass fiber membrane (GF/D, Whatman). The electrolyte was 1 M LiClO4 in DMSO. Galvanostatic tests were implemented on a battery testers (LAND CT2001A) range from 2.2 to 4.4 V (vs. Li+/Li) at room temperature under various current densities. Cycling tests with different capacity-limited modes (500 and 1000 mAh g-1) were also


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carried out. The impedance of the Li-O2 batteries was tested on a PARSTAT 2273, the testing frequency was from 10 mHz to 100 kHz and the amplitude was 5 mV. 3. Results and discussion The synthesis procedure for the direct growth of the three dimensional W-MCO-NWB on Ni foam is schematically described in Figure 1. In the first step, after a simple hydrothermal process, the interconnected web-like nanowire bundle precursors on the Ni foam substrate were obtained. In the second step, the hierarchical porous three-dimensional W-MCO-NWB was obtained by further calcination. During the synthesis process, the (NH4)2SO4 serves as the growth promoter to prompt the formation of the interconnected web-like Mn, Co-hydroxide nanowire bundles on Ni foam.14,36,37 Urea acts as the precipitant in the reaction. The reaction process can be illustrated as follows: 22 CO(NH2)2 + H2O → 2NH3 + CO2

(1)

NH3 + H2O → NH4+ + OH-

(2)

Mn2+ + Co2+ + 4OH- → Mn(OH)2 + Co(OH)2

(3)

During the annealing process: 2Mn(OH)2 + 4Co(OH)2 → 2MnCo2O4 + 6H2O

(4)

To avoid the influence of the Ni foam substrate, the MnCo2O4 powder for XRD measurement was collected from the calcined reaction precipitate. The representative XRD pattern of the sample is given in Figure 2a. No impurity peaks were observed in the product. All peaks of the diffraction can be well defined and belong to cubic spinel MnCo2O4 (JCPDS card no. 231237).38,39 This crystal phase is considered as a hybrid valence oxide presenting a spinel phase in which cobalt and manganese are both located at tetrahedral and octahedral sites, which are favourable for the composite to exhibit a high catalytic activity.22 Moreover, the XRD pattern of


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the free-standing MnCo2O4 cathode is also provided in Figure S1. The characteristic peaks of (220), (331), (511), and (440) of MnCo2O4 are distinct, although the intensity is weak under the impact of Ni foam substrate.

Figure 1. Schematic illustration of the hydrothermal synthesis of hierarchical porous threedimensional W-MCO-NWB on Ni foam

XPS tests were carried out to reveal the elemental composition and the valence state of the WMCO-NWB. As given in Figure 2b, the survey spectrum confirms the existence of O, Mn and Co elements, exhibiting the elemental composition of the MnCo2O4 sample. Moreover, the high resolution XPS spectrums of Co 2p and Mn 2p were also investigated, as shown in Figure 2c and d, respectively. The Co 2p emission spectrum (Figure 2c) is well fitted, and it can be clearly observed a Co 2p3/2 peak at 780.2 eV and a Co 2p1/2 peak at 795.3 eV. There are also two apparent shakeup satellites (labelled as Sat.) located at higher binding energy than the main 2p2/3 and 2p1/3 line.26 Similarly, the spectrum of Mn 2p (Figure 2d) indicates Mn 2p3/2 and Mn 2p1/2


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peaks at binding energies of 642.0 and 653.6 eV, respectively. By refined fitting, the fitting peaks at 641.6 and 653.2 eV confirm the existence of Mn2+, while another two peaks at 643.2 and 654.5 eV are consistent with the characteristic of Mn3+.38,40,41 Therefore, these results show the hybrid Co2+/Co3+ and Mn2+/Mn3+ coexist in the spinel W-MCO-NWB.

Figure 2. (a) XRD patterns of the prepared MnCo2O4 powder, (b-d) XPS spectra of the survey spectrum, Co 2p, and Mn 2p, respectively.

Nitrogen adsorption-desorption tests were carried out to characterize the hierarchical porous nature and specific surface areas of the three dimensional W-MCO-NWB, as given in Figure S2. A type-IV isotherm (Figure S2a) with a distinct H3 hysteresis loop is observed, which is indicative of a typical mesoporous structure. The BET is 117.1 m2 g-1, which is much bigger than those of MnCo2O4 powders,20 MnCo2O4 hollow microspheres,40 MnCo2O4 hollow nanocages27


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and MnCo2O4 nanowires21,23 in earlier reports. Additionally, the pore size distribution was also calculated as displayed in Figure S2b using the BJH method according to the desorption data. From this it is obvious that the distribution of the porous is uneven, and is mainly centred at 6.5 nm, 27.7 nm and 45.4 nm, which suggests a hierarchical porous cathode structure. Perhaps, the smaller pores were created among primary nanocrystallites, while the larger one is ascribed to the cracks between the interconnected and intercrossed MnCo2O4 nanowire bundles. It is well established that a hierarchical porous structure and a relatively large surface area is optimal for the behaviour of the gas diffusion electrode of a Li-O2 battery, which is beneficial to the transport and diffusion of ions and oxygen by offering more diffusion pathways and active sites. As a result, a considerable improvement in the electrochemical performance can be achieved. The morphology of the as-synthesized W-MCO-NWB was investigated by SEM and TEM. Figure 3a and 3b reveal the SEM images of the W-MCO-NWB sample, and it is clearly observed that the MnCo2O4 nanobundles consist of single nanowires with a diameter of about 20 nm and a length of up to few micrometers. These nanobundles or nanowires become interconnected and intercrossed with each other and further formed a 3D ordered porous web-like structure with open macroporous up to several microns in diameter. It is believed that the cross-linked threedimensional structures compared with the one-dimensional nanowires or two dimensional nanosheets can be better for promoting the transport and diffusion of electrons, ions and oxygen, and enhancing the stability of the electrode. In addition, the open microporous nature can offer plentiful space for the insoluble product, Li2O2. Moreover, Figure 3c shows the TEM image of MnCo2O4 nanobundles which consist of several nanowires, with the slender nanowire actually being composed of smaller nanoparticles. The mesopores between the nanoparticles is very clearly observed, which is identical with the results of nitrogen adsorption-desorption


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measurements. Furthermore, the spinel polycrystalline structure can be well affirmed by the SAED (selected-area electron diffraction) pattern of the W-MCO-NWB, given in the inset of Figure 3c. The calculated lattice spacing of 0.290 nm in Figure 3d is corresponding to the (220) planes of the spinel MnCo2O4.

Figure 3. (a) Low and (b) high magnification FE-SEM micrographs of the W-MCO-NWB sample; (c) TEM micrograph and SAED pattern (insert); and (d) HR-TEM micrograph of the MnCo2O4 sample.

The galvanostatic cycling test of Li-O2 batteries with the W-MCO-NWB sample at a rate of 0.1 mA cm-2 (~220 mA g-1) under a capacity limitation of 500 mAh g-1 in the voltage range of 2.2-4.4 V are presented in Figure 4a. After the first cycle, the average discharge potential was


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about 2.92V, which is approaching to the thermodynamic potential for the Li2O2 products during discharge (2.96 V). The average charging potential was around 3.46 V, ~0.54 V higher than the discharging potential, revealing a reasonable efficiency of 84.4%. Clearly, the overpotential of the Li-O2 battery with the synthesized W-MCO-NWB catalyst was comparable to that of noble metal catalytic materials like Ru/CB,42 RuO2/MnO2,43 and Pt/Au nanoparticles.44 Meanwhile the overpotential is obviously lower than those of other reported catalytic materials like Au/δMnO2,45 Au/NiCo2O4/3D-G,46 Pt/Co3O447 and MnCo2O4,21,28 under the similar testing conditions. After 300 cycles, the terminal charge voltage is still maintained below 4.4 V, and the terminal discharge voltage is still as high as ~2.7 V. The curve of terminal discharge potentials are given in Figure 4b, and the specific changes of discharge/charge voltages with time are given in Figure S3a. Furthermore, even when the capacity limitation climbed to 1000 mAh g-1, the W-MCONWB electrode still revealed excellent cycle stability over 144 cycles without any sharp decay (Figure 4c, d and Figure S3b). This suggests the desirable cycling performance of the O2 electrode with the prepared W-MCO-NWB catalyst. The effect of the W-MCO-NWB catalyst on the reaction kinetics of Li-O2 batteries was also examined, as shown in Figure 4e. Although with the elevation of discharge rate, the discharge capacity and voltage reduced, W-MCO-NWB electrodes discharged at 0.1 mA cm-2 delivered a large specific capacity of 12919 mAh g-1 with a plateau at about 2.8 V (Figure 4e). Even at higher rate of 0.2, 0.5, and 1.0 mA cm-2, the Li-O2 cell still delivered a large capacity of 10146, 7112, and 4771 mAh g-1, respectively. This suggests that our W-MCO-NWB materials have potential in in catalysing Li-O2 batteries with large energy density.


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Figure 4. (a) Galvanostatic discharge-charge curves of the W-MCO-NWB cathode at various cycles with a capacity limitation of 500 mAh g-1, at 0.1 mA cm-2 (~220 mA g-1); (b) the terminal discharge potential vs. the cycling number of the W-MCO-NWB sample; (c) discharge-charge curves of the W-MCO-NWB sample under a limited capacity of 1000 mAh g-1, at 0.1 mA cm-2; (d) the terminal discharge potential vs. the cycling number of the W-MCO-NWB sample; (e) the rate capability of the Li-O2 cell with the W-MCO-NWB electrode.

This excellent rate performance and cyclability of the W-MCO-NWB electrode is due to the three-dimensional porous network and the synergistic effect of the hybrid MnCo2O4 catalyst. Its


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hierarchical porous structures, including macropores provide enough space for insoluble Li2O2 storage and O2 diffusion and the mesopores offer sufficient catalyst active sites for the Li2O2 formation/decomposition. Furthermore, the three-dimensional network formed by interconnected and intercrossed nanowire bundles can effectively promote electron transport and alleviate the volume variation during the cycling process. Additionally, the hybrid oxidation state of MnCo2O4 can also be more catalytic activity compared with single metal oxides,25 which favours the enhancement of the performance of the MnCo2O4 electrodes. Furthermore, the free-standing W-MCO-NWB electrode without carbon and any binder can also reduce side reactions between the products and carbon, which causes the generation of irreversible carbonates at the interphase, and the degradation of binder materials by radical oxygen species, leading to poor cyclability. All these favourable factors combine to promote the electrochemical property of the Li-O2 battery with the W-MCO-NWB cathode. It should be pointed out that the charge voltage during the first ~20 cycles increases faster compared with the following cycles, either under a capacity limitation of 500 or 1000 mAh g-1, as given in Figure 4a, c and Figure S3. On the one hand, this increment is supposed to be related to the side reactions related to the electrolyte.8,15 On the other hand, it may be primarily attributed to the accumulation of incomplete degradation of the Li2O2 product inside the O2 electrodes during the first few cycles, which could decrease the conductivity of the O2 electrode and inactivate the electrocatalytic sites, resulting in a relatively rapid increase in charge potential (more evidence will be provided and discussed in the following sections). The electrochemical impedance spectrum (EIS) of the battery with the W-MCO-NWB electrode was characterized at various discharge/charge process states as given in Figure S4c. The impedance of the initial electrode is quite small, while after the first discharge the


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impedance of the electrode increased obviously. This is mainly because of the deposition of the discharge products of Li2O2 which have poor conductivity. Furthermore, the impedance of the electrode is only partly recovered after the first charge, suggesting a marked increase in the polarization, which could be ascribed to the accumulation and incomplete decomposition of the product during cycling. This is in accordance with the results that the charge potential plateau increases faster in the primary few cycles.51 XPS measurements were used for investigating the discharge products on the surface of the W-MCO-NWB electrode. As shown in Figure S4a, Li2O2 was detected after the first discharge,48 and then the peak corresponding to Li2O2 almost vanished after the first charge. More importantly, no obvious peak of by-products like Li2CO3 was discovered. This suggests the excellent catalytic performance of the W-MCO-NWB sample for the formation/decomposition of Li2O2. Furthermore, the XPS of the W-MCO-NWB electrode after cell breakdown (310th charge) was obtained (Figure S4b). From which it can be discovered that the peak corresponding to Li2O2 existed after its charge process thus indicating a residue of discharge products. At the same time, the non-reversible Li2CO3 also emerged. This indicates that the presence of Li2CO3 and the incomplete decomposition of Li2O2 deactivated the oxygen electrode, resulting in the enhancement in electrode polarization and degradation of cell performance with the cycle.48-50 The oxidation state change of the MnCo2O4 material after 100th cycling was also investigated by XPS. As shown in Figure S5, the two shakeup satellites (labelled as Sat. in Figure 2c) of the Co element is inconspicuous and the peaks of Mn 2p3/2 and Mn 2p1/2 are both changed clearly. Furthermore, the valence composition of Mn, Co elements were also estimated and listed in Table S1 and S2. It suggests that both Mn3+/Mn2+ and Co3+/Co2+ proportions increase after 100 cycles.


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SEM and TEM images of the W-MCO-NWB electrode after discharging and charging for a set number of cycles were also obtained to visually clarify the mechanism of discharge/charge processes and understand the degradation mechanism of cell performance. The Li-O2 battery was discharged and charged at 0.1 mA cm-2 under a limited capacity of 500 mAh g-1. After the 1st discharge (Figure 5a, b), the products primarily formed are thin films, which are stuck closely to the nanowires. Moreover, the open framework of the electrode, especially the macroporous structure, is still well maintained for ion transport and O2 diffusion into the inner side of the electrode. From the TEM images (Figure 5e, f), it is also intuitively observed that the discharge products of thin Li2O2 films are uniformly coated on the nanowires. Furthermore, after 1st recharge (Figure 5c and d), the hierarchical porous structure and nanowire bundles morphology were clearly regained for the porous MnCo2O4 electrode. But it also can be observed there are some undecomposed Li2O2 thin films, which have poor conductivity. This results in an increase of the impedance and polarization, resulting in a faster increment of the charge potential during initial few cycles. And the morphology of the electrode after first recharge was also characterized by TEM. From Figure S6, it can be observed that most of products had been decomposed, but there were still some bulk products remained, especially at the edge of pores formed by interconnected nanowires (Figure S6a), while no obvious products at the surface of relatively loose nanowires (Figure S6b), which are supposed to be caused by the poor contact and conductivity at the edge of pores and oxygen diffusion resistance at those small pore structure during the recharge process.


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Figure 5. SEM images of the W-MCO-NWB cathode after first discharge (a) and (b), and after first recharge (c) and (d); TEM images of pristine electrode (e) and first discharge (f), at 0.1 mA cm-2 with the capacity restricted to 500 mAh g-1.

Interestingly, after up to two months of continuous cycle testing, the micro topography of the cathode was still retained, as shown in Figure S7a and b. However, unlike the initial thin Li2O2 films, some particle-like morphology of undecomposed products which are uniformly coated on the nanowires was detected. These changes in morphologies of discharge products and performance degradations may be caused by the decrease of electrolyte level.52 Moreover, as shown in Figure S7e and f, we can clearly observe the glass fibre separator and cathode after


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300th cycles are almost completely dry, which caused the rapid increase of resistance and decrease of ionic conductivity, leading to a failure of the cell. Therefore, the volatilization of electrolyte was responsible for the decrease of the electrolyte level during long-life cycling which is a problem that cannot be ignored. Another reason for the failure of the battery may be owing to the corrosion of the Li metal,22 as shown in Figure S7c and d, a thick layer of dendritic crystals generated on the surface of the Li sheet and Li metal was almost completely powdered (Figure S7g and h). To better understand the changes of the O2 electrode, the used cathode was reassembled into a lithium oxygen battery with new lithium anode and fresh electrolyte. As shown in Figure S8, the reassembled cell can cycle over 200 cycles under a limited capacity mode, which further demonstrates that the decline of the lithium oxygen battery wasn't ascribed to the O2 cathode. These results again suggest the superior activity and stability of the W-MCONWB in catalysing the formation/decomposition of Li2O2 during cycling. Due to the poor conductivity and insolubility of the Li2O2 product, it is essential to investigate the formation processes of Li2O2 to get a better understanding of the electrochemical property of the Li-O2 batteries.14,22,53,54 Therefore, the morphology of the W-MCO-NWB at different discharge capacities of 2000, 5000, 8000, 11000 mAh g-1 and discharged to 2.2 V were characterized by ex-situ SEM, as shown in Figure 6a and Figure S9. Generally, morphologies of the Li2O2 are reliant on the current density and discharge depth.53-55 At low current density, Li2O2 particles form first as stacked thin plates, as secondary nucleation occurs new plates form and eventually a toroidal shape results. By contrast, at higher rates, the product grows as discs and toroid shapes result after copious nucleation of equiaxed Li2O2 particles.56 However, in our research, unlike the large toroidal shape particles reported in most previous reports,56-59 the discharge products of Li2O2 primarily generated are thin films, and are closely coated onto the


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surface of nanowires. When delivered a higher capacity, the thin films grow much thicker. Interestingly, when the delivered capacity is increased to 5000 mAh g-1, it can be clearly observed that some very small and thin plate-like shapes emerged on the top of interconnected nanowire bundles. These plate-like shapes become thicker and turn into large irregular polygons, and finally connect with each other into a whole film with a thickness of several tens of nanometers. This eventually leads to the macroporous structure of the W-MCO-NWB electrode being almost completely blocked. Therefore, the evolution of Li2O2 morphology and nucleation mechanism could be roughly divided into two steps, as shown in Figure 6b. In first step, active sites of the nucleation are located at the surface of MnCo2O4 nanowires and Li2O2 subsequently grows to completely cover the nanowires, which is similar with the previous report.22 However, when the thickness of the Li2O2 film coated on nanowires reaches a certain level, the active sits on the surface of nanowires is almost completely exhausted, and the top of the cross-linked skeleton consisting of nanowire bundles becomes the new nucleation site. The plate-like Li2O2 generates on new active sites and evolves into irregular polygons, and finally forms a very thick and large film, which almost completely blocks the porous structure of O2 electrode, resulting in the rapid capacity degradation. Compared with the disc or toroid-like particles, the thin film covered on MnCo2O4 nanowires can make full use of catalytic active sites, achieving higher Li2O2 filling and improved discharge capacity.14 It is also believed that the Li2O2 film with a thickness of tens of nanometers formed by secondary nucleation on the top of the intercrossed nanowire bundles can further significantly improve the discharge capacity, and the thick Li2O2 films generated from surface-mediated nucleation/thin-film is propitious to higher reversibility and rapid surface decomposition during the recharging process.60 Moreover, the

two step

nucleation mechanism can also explain the excellent cycling performance. Under a low capacity


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limitation of 500 or 1000 mAh g-1, the Li2O2 mainly forms according to the first step resulting in uniform thin film coating on the nanowires. The macroporous structure can be still maintained, and thus guarantees sufficient contact with the electrolytes and diffusion path, resulting in a good cycleability. These results also further demonstrate that the micrography of the Li2O2 product can be controlled by the shape of catalysts and suggests the great importance of the O2 cathode structure for the electrochemical performance of the Li-O2 cell.8,14,22,46 To more comprehensively understand the unique design and favourable virtues of our WMCO-NWB materials, the electrochemical performance of the current electrode and other MnCo2O4 based catalysts are summarised in Table S3. It can be intuitively observed that the WMCO-NWB cathode exhibited a very competitive performance in lithium storage capacity, cyclability and potential polarisation compared with other MnCo2O4 based catalysts. This outstanding performance of the Li-O2 cell with the W-MCO-NWB cathode can be ascribed to these favourable characteristics as follows: a) The large surface area (117.1 m2 g-1) and pore volume (0.318 cc g-1) of the W-MCO-NWB provides a significant number of electrocatalytic sites and space for high lithium storage.21,61,62 b) The inherently intercrossed web-like structure directly grown on Ni foam can efficiently promote electrical contact over the conductive substrate for faster electron transfer.18,48 And the hierarchical mesoporous/macroporous structure also can promote O2 diffusion and mass transfer. c) This free-standing cathode without any binder and carbon essentially alleviates the side reactions induced by radical oxygen intermediates, resulting in increased cycling stability of the cell.48,63


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Figure 6. a) SEM micrographs of the W-MCO-NWB electrode at various discharge capacity of 2000 mAh g-1 (a), 5000 mAh g-1 (b), 8000 mAh g-1 (c), 11000 mAh g-1 (d) and discharged to 2.2 V (e)，at 0.1 mA cm-2；b) Schematic illustration of the growth process of Li2O2.

4. Conclusions Self-supporting, carbon-free and binder-free W-MCO-NWB electrodes with a hierarchical porous structure, directly grown on nickel foam, were synthesized by a facile method. The Li-O2 cell with a W-MCO-NWB cathode showed a highly reversible capacity and excellent rate performance. The cycleability was maintained for over 300 and 144 cycles without any sharp decay under a capacity limitation of 500 and 1000 mAh g-1, respectively. The excellent


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electrochemical property is believed to result from the conductivity enhancements resulting from the hierarchical mesoporous/macroporous and interconnected web-like structure of the hybrid MnCo2O4 cathode. Together these facilitate electron transport rates and improve mass transfer rate. Furthermore, the microstructure was well maintained after long-term cycling, and the reassembled cell could run for over 200 cycles. This work clearly suggests that the main reasons for decline of the Li-O2 battery is not related to the cathode but the volatilization and decomposition of electrolyte caused the decrease of electrolyte level. An additional problem is the corrosion of lithium anode caused the powdering of Li metal during long time cycling. The two step nucleation mechanism proposed in our study can be used to explain the capacity fading and the outstanding cycleability. The present work illustrates that the good reversibility of the cathode can be achieved through electrode engineering and catalyst design, and may also provide new insights into the breakdown of Li-O2 batteries in that decreasing electrolyte loss and protection of lithium anode should be given greater attention for the development of practical LiO2 batteries. ASSOCIATED CONTENT Supporting Information XRD patterns, N2 adsorption-desorption isotherm and distribution of pore size of the WMCO-NWB sample, discharge/charge voltage profiles, EIS and XPS under different discharge/charge states, SEM and TEM after cycling, electrochemical performance of the reassembled Li-O2 battery, SEM of different discharge states, and the electrochemical performance comparison between our work and some other reports. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Kening Sun); [email protected] (Wang Sun). Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the National Natural Science Foundation of China (Grant No. 21576028, 21376001 and 21506012) for funding of this work.

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