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Hierarchical Porous Nickel Cobaltate Nanoneedle Arrays as Flexible Carbon-protected Cathodes for High-performance Lithium-oxygen Batteries Hairong Xue, Shichao Wu, Jing Tang, Hao Gong, Ping He, Jianping He, and Haoshen Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10856 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on March 11, 2016

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ACS Applied Materials & Interfaces

Hierarchical Porous Nickel Cobaltate Nanoneedle Arrays as Flexible CarbonProtected Cathodes for High-Performance Lithium-Oxygen Batteries Hairong Xue†‡, Shichao Wu‡, Jing Tang§, Hao Gong†, Ping Heǁǁ, Jianping He†* and Haoshen Zhou‡ǁǁ* †

College of Materials Science and Technology, Jiangsu Key Laboratory of Materials and Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China.

‡

Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan.

§

Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan.

ǁǁ

National Laboratory of Solid State Microstructures & Center of Energy Storage Materials and Technology, Nanjing University, Nanjing 210093, China.

*

Corresponding authors:

Prof. Jianping He, Tel: +86 25 52112900; Fax: +86 25 52112626, E-mail: [email protected], Prof. Haoshen Zhou, Tel: +81 29 8615795; Fax: +81 29 8613489. E-mail: [email protected]. ABSTRACT Rechargeable lithium-oxygen (Li-O2) batteries are consequently considered to be an attractive energy storage technology due to the high theoretical energy densities. Here, an effective binder-free cathode with high capacity for Li-O2 batteries, needle-like mesoporous NiCo2O4 nanowire arrays uniformly coated on the flexible carbon textile have been in-situ fabricated via a facile hydrothermal process followed by low temperature calcination. Due to the material and structural features, the needle-like NiCo2O4 nanowire arrays (NCONWAs) served as a binder-free cathode exhibits high specific capacity (4221 mAh g-1), excellent rate capability, and outstanding cycling stability (200 cycles). This cathode based on nonprecious mesoporous metal oxides nanowire arrays has large open

1 ACS Paragon Plus Environment


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spaces and high surface area, providing numerous catalytically active sites and effective transmission pathways for lithium ion and oxygen, and promises the abundant Li2O2 storage. The fast electron transport by directly anchoring on the substrate ensures fast electrochemical reaction process involved with the every nanowire. Furthermore, a bendable Li-O2 battery assembled by using the flexible NCONWAs as the cathode, can be able to light an LED and shows good rate capability and cyclic stability. Keywords: Mesoporous nickel cobaltate, Binder-free, Long life, Flexible cathode, Lithium-oxygen batteries INTRODUCTION Rechargeable lithium-oxygen (Li-O2) batteries are considered to be one of the most promising candidates to replace conventional lithium-ion (Li-ion) batteries as energy storage mediums.1-4 The theoretical specific energy density of Li-O2 batteries can be as high as 3505 Wh kg−1, which is almost ten times higher than that of Li-ion batteries (387 Wh kg−1).2,5 Currently, the realization of LiO2 batteries with high capacity have to adopt carbon or carbon-based materials as cathodes due to their large surface area and pore volume, which offer more catalytic active sites for oxygen reduction reaction (ORR) and storage space of the discharge product, lithium peroxide (Li2O2).6-8 For example, Super P and inverse opal carbon exhibit high capacities of 3399 and 4504 mAh gcarbon-1, respectively.6 However, the Li-O2 batteries based on carbon cathodes face several serious challenges, including large polarization and low round-strip efficiency due to the poor catalytic performance for ORR and oxygen evolution reaction (OER).9-10 In addition, carbon-involved side reactions with Li2O2 were reported. The resultant byproducts Li2CO3 and other carbonate-like species are difficult to oxidative decomposition in the charging process, further leading to extremely poor cyclic stability.11-13 These issues remain as key barriers to prevent the development of Li-O2 batteries with satisfactory performance. Therefore, carbon-alternative and carbon-protected cathode designs have been proposed as the promising strategies to mitigate the carbon-related problems.


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Carbon-alternatives, such as nanoporous gold (NPG), TiC, Ru nanoparticles supported on conductive indium tin oxide or Sb-doped tin oxides (Ru/ITO or Ru/STO), Ru grown on nickel foam and so on, have been adopted in Li-O2 batteries.14-21 NPG was first attempted and showed improved cycling stability (300 mAh gAu-1 for 100 cycles).15 TiC electrode exhibits good reversible formation/decomposition of Li2O2 (350 mAh gTiC-1 for 100 cycles).16 Ru/ITO and Ru/STO both demonstrated good cycling performance (1.81 mAh cm−2 and 750 mAh gRu/STO-1 for 50 cycles, respectively).17-18 Nanoporous Ru directly grown on nickel foam as binder-free cathode delivered specific capacity of 3720 mAh gRu-1 and cycling stability for 100 cycles (1000 mAh gRu-1).19 RuO2 nanosheets and spheres were reported to achieve 50 and 100 cycles with full discharge and charge capacity of ~800 mAh gRuO2-1.14,20 For carbon-protected cathode design,7.22 Jian et al. attempted to introduce RuO2 shell covering on the surface of carbon nanotube as cathode. The core-shell structure preventing carbon-involved side reactions enabled the battery showed high round-trip efficiency (~79%), good rate and cyclic performance (300 mAh g-1 for 100 cycles).7 Although the cyclic stability was significantly improved by carbon-alternative and carbon-protected cathode designs, the discharge and charge capacities were severely restricted due to the high formular weight of these materials. In addition to the low capacity, fancy price of noble metals or poor catalytic activity of other non-noble metals and low electronic conductivity of most metal oxides force it necessary to explore promising cathode designs with low cost, strong catalysis, high capacity and long cycle life. NiCo2O4 with good catalytic activity, low cost and natural abundance, has been extensively used in the fields of supercapacitors,23-24 catalysts,25-26 and lithium ion batteries.27-28 Besides, because of the better bifunctional catalytic activity of ORR and OER and particularly high electron conductivity (10-3 to 10-2 S cm-1),29-32 NiCo2O4 was also attempted as cathodes for Li-O2 batteries. Although the reduced discharge/charge overpotentials and large capacity were reported,33-35 the addition of conductive carbon black and non-conductive polymer binder leaded to the limited cycling abilities (no more than 50 cycles). Herein, we in-situ fabricated needle-like mesoporous NiCo2O4 nanowire



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arrays (NCONWAs) as a binder-free cathode for high-performance Li-O2 batteries with high specific capacity (4221 mAh g-1), excellent rate capability and outstanding cycling stability (200 cycles). Mesoporous NiCo2O4 nanowires are composed of numerous nanoparticles with highly crystalline, supplying plenty of catalytical active sites, promoting the oxygen diffusion and electrolyte infiltration. Large open spaces created by the nanowire arrays make more NiCo2O4 nanocrystals easily contact with the oxygen and electrolyte, provide effective transmission pathways for lithium ion and oxygen, and more importantly, meet the requirement of abundant Li2O2 storage. Additionally, NCONWAs uniformly covering on carbon textile isolates the contact between Li2O2 and carbon, prevents the related side reactions and thus promotes the cycling stability. Therefore, the mesoporous NCONWAs cathode for the Li-O2 batteries shows higher specific capacity, better rate capability and cyclic stability. Moreover, this bendable cathode exhibits potential application for flexible electronics. RESULTS AND DISCUSSION

Figure 1. (a) Schematic illustration of the formation of NCONWAs cathode; (b) XRD patterns and (c) XPS spectra of NiCo2O4 nanowires scraped down from the substrate. 4 ACS Paragon Plus Environment

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A schematic overview of the fabrication processes for the electrode architectures is illustrated in Figure 1a. Carbon textile is selected as the current collector because of its light weight, high conductivity and flexibility together with large surface and porosity. Under hydrothermal condition, the suitable pH level of the precursor solution can be adjusted by the hydrolysis of urea, which is beneficial to the growth of the NiCo-precursor nanowire arrays on carbon textile. 24,36 The following equations can describe this synthesis mechanism: CO(NH2)2 → C3H6N6 + 6NH3 + 3CO2

(1)

NH3 + H2O → NH4+ + OH−

(2)

Ni2+ + 2Co2+ + 6OH− → NiCo2(OH)6

(3)

After annealing treatment in air, the NiCo-precursor is converted to spinel NiCo2O4 through an oxidation reaction process，which is described by following equation: NiCo2(OH)6 + 1/2O2 → NiCo2O4 + 3H2O

(4)

The NiCo2O4 nanowires powders are scratched from substrate to reduce the strong impact of the substrate for X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. As shown in Figure 1b, all of the diffraction peaks of the samples could be well indexed to the spinel NiCo2O4 phase (JCPDS card no. 20-0781). The crystal structure of NiCo2O4 is demonstrated in the inset of Figure 1b. By using the Scherrer formula, based on the half-peak breadth of the (311) diffraction peak,35 the average crystal size of NiCo2O4 nanowires was calculated to be approximately 8.6 nm, suggesting that the nanowires consist of nanoparticles (in accordance with the TEM results). The XPS spectrum further confirms the more detailed elemental composition and chemical valence of the NiCo2O4. As is shown in Figure S1 (Supporting Information), a survey scan shows there are four elements (Ni, Co, O and C) within the NiCo2O4 nanowires. The peak located at 529.7 eV is assigned to O element in oxide, and the peak of C1s should be assigned to the carbon textile substrate. As shown in Figure 1c, the Ni2p spectrum is well fitted with two spin-orbit doublets ascribed to Ni3+ and Ni2+, and two shakeup satellites (indicated as “Sat.”), by using the the Gaussian



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fitting method.37 Similarly, the Co2p is also well fitted with two spin-orbit doublets ascribed to Co2+ and Co3+, and one shakeup satellite.38 These results of the XPS spectrum are in good agreement with the previous research on NiCo2O4.37,38 Base on the TG analysis (Figure S2, Supporting Information), the NiCo2O4 mass loading in the NiCo2O4 nanowire arrays growth on carbon textile cathode is calculated to be ~0.45 mg cm-2, the mass fraction of NiCo2O4 is 3.7 wt%.

Figure 2. (a) Low and (b) high magnification SEM images of the crystalline NCONWAs cathode; (c) TEM, (d) HRTEM images and SAED pattern (inset) of the NiCo2O4 nanowires scraped down from the substrate; (e) EDX mapping of the NCONWAs; (f) TEM image of the urchin-like NiCo2O4 microspheres. Field-emission scanning electron microscopy (FESEM) was used to examine the morphologies of the NCONWAs. The NiCo-precursor (Figure S3, Supporting Information) grown on the the carbon microfiber core clearly displays the well-established needle-like nanowire array structure, forming a large-scale conformal coating. Note that the conical needle-like NiCo-precursor nanowires have very


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smooth surfaces. As shown in Figure 2a and b, numerous NiCo2O4 nanowires are uniformly and vertically grown on the carbon textile, forming relatively aligned nanowire arrays on a large scale. After annealing conversion into spinel NiCo2O4, the NiCo2O4 nanowires still keep the nanowire arrays structure of the NiCo-precursor NWAs. Typical nanowires have the lengths of about several micrometers with diameters around 80-100 nm. As shown in Figure S4 (Supporting Information), the cross-sectional SEM image of a broken NCONWAs further reveals that the carbon fiber can be perfectly protected due to coating with the nickel cobaltate nanoneedle arrays layer. A detailed TEM study was performed to further distinguish the structure of the NCONWAs. As can be seen from Figure 2c and d, the needle-like NiCo2O4 nanowire is highly porous, composed of many small nanoparticles of 7-10 nm in size. The pore structural characteristics of NiCo2O4 nanowires were determined

by

Nitrogen

isothermal

adsorption-desorption

measurements.

The

nitrogen

adsorption/desorption isotherms of NiCo2O4 nanowires in Figure S5a (Supporting Information) show a typical IV shape with H1-type hysteresis loops, indicating a typical mesoporous structure.39 Moreover, when the relative pressure is close to the saturated vapor pressure, the adsorption/desorption curves still incline to go up, revealing some slit-like pores within the NiCo2O4 nanowires.40 The Brunauer–Emmett–Teller (BET) surface area of the NiCo2O4 nanowires is close to 90 m2 g-1. The pore-size distribution curves of the NiCo2O4 nanowires by using Barrett-JoynerHalenda (BJH) model are shown in Figure S5b (Supporting Information), calculated from the desorption branch. A unimodal peak is distributed around 11 nm, which is consistent with the TEM results and further confirms the mesoporous structure of NiCo2O4 nanowires. The formation of mesoporous structure could be related to the release of H2O and gases by the decompositionoxidation of intermediates during calcining process.41 Figure 2d clearly presents a lattice spacing of 0.468 nm, corresponding to the theoretical interplane spacing of the (111) planes for spinel NiCo2O4. The SAED pattern shows well-defined rings indexed to the spinel NiCo2O4 phase, indicating the polycrystalline characteristics of NiCo2O4 nanowires. Moreover, energy dispersive X-ray (EDX)



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spectrometry mapping shown in Figure 2e, unambiguously confirms that the NiCo2O4 nanowires completely cover the surface of carbon textile. By contrast, Figure 2f shows the TEM image of an individual urchin-like NiCo2O4 microsphere (NCOMS) prepared following the similar method without using substrates. NCOMS is constructed by a large number of nanowires. The unique porous structure and core-shell design are expected to enable NCONWAs cathode exhibiting high discharge capacity and superior cycling ability for Li-O2 battery.

Figure 3. (a) Full discharge and charge profiles of the Li-O2 battery with NCONWAs and NCOMS cathode in the first cycle; (b) Electrochemical impedance spectra of the NCONWAs and NCOMS cathodes before first discharge; (c) Schematic drawing of the different electrode architectures including conventional NCOMS electrode and binder-free flexible NCONWAs electrode. The urchin-like NCOMS and binder-free NCONWAs sample were evaluated as cathodes in rechargeable Li-O2 batteries, using 1 M lithiumbis(trifluoromethane-sulfonyl)imide (LiTFSI) in


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tetraethylene glycoldimethyl ether (TEGDME) as electrolyte. The urchin-like NCOMS cathode was prepared by coating a paste of the NCOMS sample (50 wt%), super P (45%) and polytetrafluoroethylene (PTFE, 5 wt%) onto carbon paper. The carbon textile and carbon paper exhibit very low discharge capacity and almost no charge capacity (Figure S6, Supporting Information). The first full discharge and charge profiles of the NCOMS and NCONWAs cathodes at 200 mA g−1 within a cut-off voltage window of 2.3 to 4.3 V under O2 atmosphere are shown in Figure 3a. The discharge capacity of the NCONWAs cathode is as high as 4221 mAh g-1, which is comparable to carbon based cathodes and is much higher than the NCOMS cathode (3409 mAh g -1). The discharge plateau is at ~2.78 V, corresponding the overpotential of 0.18 V. For the charge process, a higher coulombic efficiency of about 100% is achieved for the NCONWAs cathode due to the low charge overpotential of 0.86 V. In contrast, the NCOMS cathode shows a discharge and charge overpotential of 0.36 and 0.97 V, resulting in a lower coulombic efficiency close to 84%. Electrochemical impedance spectra (Figure 3b) shows the Nyquist semicircle of the NCOMS cathode is much bigger than that of the NCONWAs cathode, indicating the high charge-transfer resistance due to the addition of the polymer binder. The significantly different electrochemical performances for the urchin-like NCOMS and binder-free NCONWAs cathodes are attributed to their electrode architecture. Figure 3c shows the configuration of the NCOMS cathode, composed of NiCo2O4 microspheres, polymer binder and conductive carbon on carbon paper, the same as other conventional binder-enriched electrodes in Li-O2 batteries. The addition of the insulated binder and non-uniform distribution of the conductive carbon increase additional undesirable interparticle resistance, destroy the fast electronic transmission between current collector and electrocatalytic materials, and prevent efficient diffusion of oxygen and infiltration electrolyte. This result can be confirmed by the electrochemical impedance spectra, as shown in Figure 3b. The binder-free NCONWAs cathode exhibits superior electrochemical reaction pathways in Li-O2 batteries due to the following features: 1) The NCONWAs in-situ coating on the carbon textiles ensures intimate



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contacts and effective electron transport between the carbon textiles and the NiCo2O4 nanowires; 2) The NiCo2O4 nanowire arrays completely covered the entire surface of the carbon microfibers, preventing direct contact between carbon and Li2O2, avoiding or reducing the formation of Li2CO3; 3) Mesoporous NiCo2O4 nanowires consist of numerous highly crystalline nanoparticles, supplying plenty of catalytical active sites, promoting the electrolyte infiltration and oxygen diffusion; 4) Large open spaces formed by the nanowire arrays can make more NiCo2O4 nanocrystals easily contact with the electrolyte and oxygen, provide effective transmission pathways for lithium ion and oxygen, and more importantly, provide more void volume for the abundant discharge product Li2O2 deposition; 5) There are no binders or any conductive carbon black in this unique electrode architecture, avoiding some detrimental side reactions, such as the possible formation of Li2CO3 between carbon and Li2O2, LiF and LiOH produced by the dehydrofluorination reaction of the binder. These factors are helpful to enhance the effective diffusion of ion, electron and oxygen and avoid the secondary reaction in the NCONWAs cathode architecture, thus leading to remarkable specific capacity, good rate capability and superior cycling stability.


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Figure 4. (a,c) The discharge and charge profiles of the NCONWAs and NCOMS cathodes, respectively; (b,d) Galvanostatic discharge and charge curves of the NCONWAs and NCOMS cathode at different current densities (100, 200, 500 and 1000mAh g-1), respectively; (e) Cyclic stability and the variation of the terminal charge and discharge voltages over 200 cycles for the Li-O2 battery with NCONWAs cathode. All the discharge and charge processes are examined with a limited capacity of 1000 mAh g-1. The rate capability and cycling performance of the binder-free NCONWAs and NCOMS cathodes were further examined with a limited capacity of 1000 mAh g-1. As shown in Figure 4a, the NCONWAs cathode shows very small polarization values of 0.18 V and 0.7 V for the discharge and charge overpotentials, respectively, suggesting an excellent catalytic activity for ORR/OER. More importantly, the NCONWAs cathode exhibits impressive cycling stability in Li-O2 batteries. There is



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almost no obvious variation and decay of the specific capacity in the discharge and charge profiles over 200 cycles (Figure 4e). The discharge and charge terminal voltages maintain well. After 200 cycles, the discharge terminal voltage decreases from 2.8 V to 2.6 V and the charge terminal voltage increases from 4.1 V to 4.3 V, implying excellent cycling stability. The rate capability of the NCONWAs cathode was evaluated at different current densities (100, 200, 500 and 1000mAh g-1), as shown in Figure 4b. When the current density increases from 100 mA g−1 to 1000 mA g−1, the charge and discharge overpotentials increase from 0.55 to 0.94 V and 0.16 to 0.26 V, respectively, whereas the coulombic efficiencies are still 100%. The results indicate a good rate capability of LiO2 batteries. In contrast, for NCOMS cathode in Figure 4c and d, the Li-O2 battery demonstrates poorer cycling stability and rate capability. The discharge voltage plateau shows obvious decrease from the 40th cycle and the charge terminal voltage reaches to the cut-off voltage 4.3 V even from the 10th cycle, leading to the dramatically decreased coulombic efficiency. The discharge capacity at 1000 mA g-1 is reduced to only 800 mAh g-1. We also compared this work with the other nonprecious metal/metal oxide-based, precious metal/metal oxide-based, and carbon-alternatives cathode materials from some published literatures, as shown in Table S1 (Supporting Information). Compared with those cathode materials, the unique binder-free needle-like NiCo2O4 nanowire arrays cathode prepared in this work still shows the better electrochemical performance than the most of the cathode materials reported in previous literatures. Consequently, the significantly enhanced rate capability and cycling stability are attributed to the advantages of NCONWAs structure. In this optimized electrode structure, low interface impedance, fast electron transport and large channels for Li2O2 storage improve rate capability especially at high current density. No insulated binder in the cathode avoids additional side reactions, which could result in the increased overpotentials and capacity decay during round-trip cycling. Most importantly, as shown in Figure 2, the entire surface of the carbon microfibers are completely covered with the NiCo2O4 nanowire arrays, avoiding or reducing the carbon-involved side reactions with Li2O2, thus leading to the excellent cycling stability.


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Figure 5. (a) XRD pattern of NCONWAs cathode after the first discharge and recharge; (b) The discharge and charge curves of the NCONWAs cathode in the first cycle at the current density of 200 mA g−1; (c) SEM images of the NCONWAs cathode at different states in accordance with the point (I-IV) in (b), respectively. XRD and SEM measurements were utilized to primarily identify the discharge products of Li-O2 battery with NCONWAs cathode. Figure 5a shows the XRD patterns of the NCONWAs cathode after the first discharge and charge, in accordance with the stage- III and IV in Figure 5b. For the discharged cathode, there are two distinctly new diffraction peaks at around 2θ =32.8° and 34.8°, which can be indexed to (100) and (101) diffraction of discharge product Li2O2, respectively.42,43 When the battery is recharged to 4.3 V, the diffraction peaks of Li2O2 disappear, indicating that the Li2O2 is decomposed in the charging process. The reversible Li2O2 formation/decomposition is further confirmed by SEM shown in Figure 5c, corresponding to the stage I - IV in Figure 5b, 13 ACS Paragon Plus Environment


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respectively. Figure 5c exhibits the fresh cathode composed of uniform nanowires and large channels formed between micro-sized NCONWAs. When the cathode is discharged to the capacity of 1000 mAh g-1 at stage-II, the discharge products (Li2O2) with macro-sized irregular plate-shape structure are deposited on the surface of the NCONWAs and partly fill in the large channels. After fully discharging to 2.3V (stage-III), the NCONWAs are uniformly covered by a large amount of typically toroid-shaped Li2O2 particles (~400 nm), resulting in the reduced porosity arrays. Large open spaces formed by the nanowire arrays make more NiCo2O4 nanocrystals easily contact with the electrolyte and O2, offer the effective transmission path for Li+ and O2, and meet the requirement of abundant Li2O2 storage, leading to high specific capacity of 4221 mAh g-1. After charging back to 4.3V (stageIV), the Li2O2 is rarely observed, suggesting high reversibility of this binder-free NCONWAs cathode. In addition, Figure S7a (Supporting Information) shows the XRD patterns of the discharged and charged NCONWAs cathode in the 50th cycle. The discharge product after the 50th discharge can be confirmed by the characteristic diffraction peaks of Li2O2. It reversibly disappears after the 50th charge, evidenced by the absence of its characteristic diffraction peaks. This result is in agreement with the SEM observations shown in Figure S7b and c (Supporting Information), which further indicates the excellent reversible formation and decomposition of Li2O2. The good difunctional catalytic activity of ORR and OER for NCONWAs could be attributed to the highly efficient active sites and increased electrical conductivity, resulting from inserting Ni cations into the octahedral sites of the spinel lattice.35 Furthermore, the mesoporous NCONWAs are consisted of numerous highly crystalline nanoparticles, leaving a high surface area to provide numerous catalytic active sites, benefiting the efficient diffusion of both Li+ and O2.


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Figure 6. Schematic illustration and electrochemical performance of a bendable Li-O2 battery assembled using flexible NCONWAs cathode. The obtained NCONWAs cathodes exhibit high specific capacity, good rate performance, and outstanding cycling stability. In this paper, we further built a bendable Li-O2 battery by using the binder-free NCONWAs cathode, which shows a potential application of this bendable cathode for flexible electronics. The structure of this bendable battery is shown in Figure 6, assembled of the NCONWAs cathode, separator, LiTFSI-TEGDME electrolyte and Li foil anode. A digital photograph of NCONWAs cathode shows the good flexibility, which can be folded and flexed. The thin and bendable final packaged battery like a thick paper is also displayed, which exhibits good rate capability and cycling stability. In addition, whether it is flat or bended, a green LED could be lighted all the time (The detailed features are shown in Supporting Video). Benefiting from the excellent electrochemical performances and bendable properties, the NCONWAs cathode has potential applications in flexible and wearable electronics devices, such as roll-up display, wearable sensors, and irregularly shaped energy storage devices. CONCLUSION



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Mesoporous NiCo2O4 nanowires arrays in-situ grown on the carbon textile (NCONWAs) are successfully fabricated, using a facile hydrothermal process followed by calcining treatment, which are directly served as binder-free cathode for Li-O2 batteries. In this electrode design concept, the expedite electronic transmission between each NiCo2O4 nanowire and the carbon textile substrate, and facile ion and oxygen diffusion, ensure the fast electrochemical reaction process involved with the every nanowire. The large open spaces consisted of the adjacent nanowires promise the abundant Li2O2 storage. NiCo2O4 NWAs covering the entire surface of carbon textile mitigates carbon-related issues. As a cathode for Li-O2 batteries, the NCONWAs exhibits reversible high capacity of 4221 mAh g−1 at 200 mA g−1 and excellent cyclic stability (No capacity decay after 200 cycles with the limited capacity of 1000 mAh g -1). A bendable Li-O2 battery with a good electrochemical performance is designed and assembled, in which the NCONWAs can be directly served as a flexible cathode, exhibiting its potential applications in flexible and wearable electronics devices. EXPERIMENTAL METHODS Mesoporous NiCo2O4 nanowire arrays (NCONWAs) cathode: Carbon textiles were purchased from Shanghai Hesen Corporation. Phenol (C6H5OH), cobalt nitrate (Co(NO3)2·6H2O), urea (CO(NH2)2), and nickel nitrate (Ni(NO3)2·6H2O) were purchased from Shanghai Chemical Corporation. All chemicals were used as received without any further purification. Carbon textile as a substrate was sequentially cleaned by acetone, distilled water, and ethanol solution, with the assistance of sonication for 30 min each. In a typical process, a transparent pink solution was formed by dissolving 0.36 g of urea, 0.29 g of Co(NO3)2·6H2O, and 0.145 g of Ni(NO3)2·6H2O in 40 mL of mixed solution with ethanol and H2O (V : V = 1 : 1) at room temperature. The obtained solution was transferred into a 60ml Teflon-lined stainless steel autoclave and then immersed the treated carbon textile (2 cm × 1 cm) into it. Then, the autoclave was heated to 90 °C and kept at this temperature for 16 h in a conventional oven.) After repeatedly rinsing with distilled water and ethanol combined with sonication, the product was dried at 60 ℃ in air. Finally, the NiCo-precursor NWAs on the carbon


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textile converted into well crystallized NiCo2O4 NWAs by annealing at 300 °C for 6 h. The synthesis of NiCo2O4 microspheres was similar to the NiCo2O4 NWAs without adding carbon textile. Materials Characterization: The X-ray diffraction (XRD) was conducted on Bruker D8 advance diffractometer using Cu Kα radiation to investigate the crystal structure of the each sample. The chemical compositions were characterized by X-ray photoelectron spectroscopy (XPS) analysis (Perkin-Elmer PHI 550) under Al Kα radiation. The weight retention of NiCo2O4 was calculated by Thermogravimetric/Differential Thermal Analysis (TG/DTA) (PerkinElmer TGA 7) under flowing air from room temperature to 900 ℃. The field-emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), and high-resolution transmission electron microscope (HR-TEM) conducted on Hitachi S-4800, FEI Tecnai-20, and JEOL JEM-2100, respectively, were used to observe the morphology and microstructural of the as-prepared samples. The N2 adsorption– desorption measurements were carried out by using a surface area analyzer (ASAP-2010). The specific surface area of the NiCo2O4 nanowires was calculated by Brunauer-Emmet-Teller (BET) method. The pore-size distribution derived from the desorption isotherm was estimated by using Barett-Joyner-Halenda (BJH) method. Electrochemical Measurement: The NCONWAs directly acted as cathode without any ancillary materials, hence the capacity of NCONWAs was calculated based on the loading weigh of NiCo2O4 excluding the current collector (carbon textile). For electrochemical measurements of NiCo2O4 microspheres, this electrode was prepared by mixing 50 wt.% active materials with 45 wt.% carbon black (Super P) and 5 wt.% binder (polytetrafluoroethylene (PTFE, 15 wt%)). Then the obtained electrode was pasted on the carbon paper (as current collector) followed by drying at 100 °C in the vacuum oven for 12 h. The capacity of NCOMS was calculated based on the weight of the whole electrode excluding the current collector (carbon paper). The Li-O2 battery was assembled by using the coin cell in an argon-filled glove box. The NCONWAs directly used as cathode was separated from the Li foil anode by a glass-fiber separator, impregnated into 1 M LiTFSI in TEGDME



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electrolyte. The electrochemical measurements were performed on Hokudo Denko Charge/Discharge instruments at 25 ℃ and in the pure O2 atmosphere. The AC impedance spectra was measured from 10−2 to 106 Hz using a Solartron 1260 frequency response analyser combined with a Solartron 1287 potentiostat. ASSOCIATED CONTENT Supporting Information Additional details are available, including the XPS spectra, TG and DTA curves, SEM and FESEM images, N2 adsorption-desorption isotherm and pore-size distribution curve, full discharge/charge curves, XRD patterns, and comparison of electrochemical performance between this study and some other previous literatures. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * Prof. Jianping He, Tel: +86 25 52112900; Fax: +86 25 52112626, E-mail: [email protected], * Prof. Haoshen Zhou, Tel: +81 29 8615795; Fax: +81 29 8613489. E-mail: [email protected]. ACKOWLEDGMENTS The authors appreciate the financial support from the National Natural Science Foundation of China (51372115, 11575084), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the National Basic Research Program of China (973 Program, 2014CB932302) and National Natural Science Foundation of China (21373111, 21403107). REFERENCES [1] Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature, 2001, 414, 359-367 [2] Bruce, P.G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S Batteries with High Energy Storage. Nat. Mater. 2012, 11, 19-29.


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Hierarchical Porous Nickel Cobaltate Nanoneedle ... - ACS Publications

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