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Amorphous ZnO Quantum Dots (QDs)/Mesoporous Carbon Bubble Composites for High-perforemance Lithium-ion Battery Anode Zhiming Tu, Gongzheng Yang, Huawei Song, and Chengxin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13113 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 15, 2016

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ACS Applied Materials & Interfaces

Amorphous ZnO Quantum Dots (QDs)/Mesoporous Carbon Bubble Composites for High-Perforemance Lithium-Ion Battery Anode

Zhiming Tu, Gongzheng Yang, Huawei Song and Chengxin Wang* The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, People’s Republic of China

*Correspondence and requests for materials should be addressed to C. X. Wang. Tel & Fax: +86-20-84113901, E-mail: [email protected] 1

ACS Paragon Plus Environment


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Abstract Due to its high theoretical capacity (978 mA h g-1), natural abundance, environmental benignity, and low cost, Zinc oxide is regarded as one of the most promising anode material for lithium ion batteries. A lot of research has been done in the past years. However, there is hardly any research of amorphous ZnO for LIB’s anode has been reported, despite that the amorphous one could have superior electrochemical performance due to the isotropyic nature, abundant active sites, better buffer effect and different electrochemical reaction details. In this work, we develop a simple route to prepare amorphous ZnO quantum dots (QDs)/mesoporous carbon bubble composite. The composite consists of two parts: mesoporous carbon bubbles as a flexible skeleton and monodisperse amorphous zinc oxide QDs (smaller than 3 nm) encapsulated in an amorphous carbon matrix as a continuous coating tightly anchored on the surface of mesoporous carbon bubbles. Benefiting from the abundant active sites, amorphous nature, high specific surface area, buffer effect, hierarchical pores, stable interconnected conductive network, and multidimensional electron transport pathways, the amorphous ZnO QDs/mesoporous carbon bubble composite delivers a high reversible capacity of nearly 930 mA h g-1 (at current density of 100 mA g-1) with almost 90% retention for 85 cycles, and possess a good rate performance. This work opens the possibility to fabricate high-performance electrode materials for LIBs, especially for amorphous metal oxides-based materials.

Keywords: Zinc oxide, amorphous metal oxides-based materials, quantum dots, lithium ion batteries, high-performance electrode.

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Introduction Under the context of urgent energy crisis and environmental degradation problems, Li-ion battery technology owes to its ability of potentially providing a solution of cheap and sustainable energy supply for various portable electrical appliances and electric vehicles, especially as they can be used to store energy from sustainable sources such as the wind and solar power, is attracting more and more interest.1 To meet the imperative demand of high-performance batteries, researchers have been looking for better electrode materials. Due to its high theoretical capacity, natural abundance, environmental benignity and low cost, metal oxide2 (TiO23, ZnO4, GeO25, SnO26, NiO7, Co3O48, and Fe2O3/Fe3O49) is regarded as one of the most promising candidate of the commercial graphite, which has been widely used for the current LIBs anodes with a low theoretically capability of 372 mA h g−1.10 ZnO delivers a theoretically capacity of 978 mA h g−1, which is nearly three times higher than that of graphite (372 mA h g−1), performing as (1) (2)11 However, the inherent poor electrical conductivity and large volume expansion (about 150 %)11-12upon Li+ insertion/extraction process, which may give rise to pulverization of electrode and destroy the electrical contact13, lead to a poor rate performances and abrupt capacity fading, when it used as Li-ion battery anode. In order to circumvent these disadvantages, many strategies have been proposed and investigated. Different morphology design studied earlier has mainly focused on nanocrystallization and porous structure14-15, such as nanosized ZnO nanotubes16, flower-like ZnO nanostructures17,mesoporous ZnO nanosheets, and ZnO nanorod arrays18, improved the performance both in rate and cycling stability compared with bulk materials, since they may effectively offer the vacant space to alleviate the volume variation, larger electrode/electrolyte contact area to realize higher rate performance, shorten Li+ diffusion distance and shorten path lengths for electronic transport14. Unfortunately, the circulation ability still need to be further improved. In the study of recent years, various ZnO/C composites have been researched, Such as, ZnO\graphene composite4, ZnO/C microboxes19, ZnO/mesoporous carbon nanocomposite20, and ZnO\C hybrid materials21. These carbon-containing composites not only bring better electrical conductivity and good interparticle electrical contact but also prevent the aggregation and pulverization of the nanosized 3



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particles. Moreover, the thin carbon layers also possess some elasticity to buffer the strain of volume change upon the consecutive Li+ insertion/extraction22. Although these studies have solved many problems, from the point of practical application, it is still a challenge to find stable and high-capacity ZnO electrodes. Amorphization is another path way for designing high-performance electrode materials.23 A number of studies have proved that some amorphous anodes have a superior performance than that of corresponding crystal ones, which is derived from its properties of isotropyic nature, abundant active sites, better buffer effect and different electrochemical reaction details.5, 23-31 For crystal materials, the insertion of lithium ion is anisotropic, which is ascribed to distinction of periodicity of the atomic arrangement and degree of density along different direction of the lattice; however, for amorphous materials, the insertion of lithium ion could be isotropic, thus it is more effective to absorb the volume changes during the successive Li+ insertion/extraction for amorphous materials. In addition, it is worth mentioning that the crystal→amorphous phase change occured by electrochemically-driven solid-state amorphization (ESA) during the first lithium intercalation process may be circumvented effectively11-12. Through the preparation of amorphous materials instead of crystalline ones, some materials deliver a good electrochemical performance in Li-ion battery. Zhang et al.31 reported SSL ZnO/C NFs composites which delivers a high reversible capacity of 813.3 mAh g-1 at 100 mA g-1 with a good rate performance. Jin et al.5 succeed in synthesis of the vertically aligned graphene@amorphous GeOx sandwich nanoflake which shows a stable capacity of 1008 mAh g-1 for 100 cycles (with capacity retention of 96%). However, in previous works, the ZnO electrodes were commonly based on crystalline ZnO nanoparticles, research of amorphous ZnO for LIB’s anode has been rarely reported.24 Taking these into consideration, in this work, we develop a facile CVD method to prepare amorphous ZnO QDs/mesoporous carbon bubbles (MPCBs) composite (schematically illustrated in Figure 1) with monodisperse zinc oxide quantum dots(smaller than 3 nm)encapsulated in an amorphous carbon matrix anchored on the shell of mesoporous carbon microbubbles without agglomeration. When used for Li-ion battery anodes, the amorphous ZnO QDs/MPCBs composite exhibit a superior Li storage performance.

Results and discussion Figure 2(a) shows the X-ray diffraction (XRD) patterns of amorphous ZnO quantum dots/mesoporous carbon bubbles composite. There is no diffraction peak can be detected in the XRD pattern of the composite, 4


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revealing no legible crystalline phase. X-ray photoelectron spectroscopy (XPS) was employed to further determine the valence state of Zinc in the amorphous ZnO QDs/MPCBs composite. The ZnLMM spectra (Figure 2(c)) and Zn2p spectrum (Figure 2(d)) show a typical ZnO profile. The valence state of Zinc in the composite is confirmed by the two binding energy peaks located at 1045.4 eV (Zn2p1/2) and 1022.3 eV (Zn2p3/2), which is correspond to core levels of the zinc (II) ions. And the peak record in the ZnLMM spectra located at 986.6 eV is also in accordance with ZnO profile. The existence of ZnO is further confirmed by the FT-IR spectrum, in which the absorption peak at 503 cm-1 corresponds to the characteristic vibration of Zn-O. Some characteristic absorption peaks of oxygen functional groups also were found in the spectrum, corresponding the stretching vibrations of epoxy C-O-(H) and C-O-(C) located at 1083 cm-1, hydroxyl groups (–OH) located at 1412 cm-1 and 3384 cm-1, and the C = C band located at 1570 cm-1, respectively, suggesting that carbon

matrix

contains abundant functional groups.

The

heterogeneous

functional groups may also bring abundant active sites availing for ions adsorption, thus may provide an extra capacity for electrode materials.32-35 The morphology and structure of the amorphous ZnO QDs/MPCBs were characterized by scanning electron microscopy (SEM, FEI/Philips XL30) and transmission electron microscopy (TEM). Figure 3 shows scanning electron microscope (SEM) images of pristine MPCBs (Figure 3(a)) and amorphous ZnO QDs/MPCBs composite (Figure 3(b)). In the image of pristine MPCBs (Figure 3(a)), we can see that transparent bubble-like hollow micro-spheres with homogeneous and discrete spherical morphology MPCBs have been obtained successfully. After synthesis of amorphous ZnO/carbon bubble composite, the SEM image was shown in Figure 3(b). As presented in the SEM image, the composite inherits the bubble-like structure and possess thicker shell after synthesis of amorphous ZnO QDs. The structure details of amorphous ZnO QDs/MPCBs were illustrated by TEM micrographs. As shown in Figure 4, monodisperse zinc oxide QDs (smaller than 3 nm) encapsulated in an amorphous carbon matrix as a continuous coating tightly anchored on the surface of mesoporous carbon bubbles were prepared successfully. In order to further determine the structure of ZnO QDs in the as-prepared sample, selected area electron diffraction (SAED) was employed. It only shows a dispersed halo, which indicates the ZnO QDs possess amorphous structure. From the TEM mounted EDS spectrum in Figure 4(b), we can see that only C, Zn, and O exist in the hybrid composite. In addition, combining the XRD, FT-IR spectra and XPS survey spectrum of the composite, it can be certainly determined that, the monodisperse ZnO QDs with amorphous structure has been prepared successfully. The amorphous carbon matrix and a low temperature annealing process may the crucial factor 5



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of preventing crystallization and agglomeration of ZnO QDs. In the preparation of amorphous ZnO QDs/CBs composite, the ZnO QDs is monodispersed and encapsulated in an amorphous carbon matrix, which means that each ZnO QD is separated by amorphous carbon matrix. Since the surface atomic number of QDs(smaller than 3 nm)takes a grate proportion of its total atomic number, and take this in mind, the huge interface and the inter-atomic forces between the ZnO and C may hinder the long-term orderly arrangement of atoms with a low temperature annealing process. Thus the amorphous state ZnO QDs was successfully obtained. Element mapping also has been employed to investigate precise element distributions of ZnO and C, as shown in Figure 5(A)–(H), Zn and O elements was uniformly, continuously distributed in the carbon matrix. All in all, combining all of the above characterizations, the composite consists of two parts: mesoporous carbon bubbles as a flexible skeleton and monodisperse amorphous zinc oxide QDs (smaller than 3 nm) encapsulated in an amorphous carbon matrix as a continuous coating tightly anchored on the surface of mesoporous carbon bubbles. Such a unique structure endows the material with ultrahigh capacity and stability, when employed as an anode material in Li ion batteries. Thermogravimetric analysis (TGA) was further employed to determine the content of carbon and ZnO. As shown in Fig. 6, a significant weight loss of 50 wt% is recorded at about 400 °C, which indicates that the content of carbon in the amorphous ZnO QDs/MPCBs is 50 wt%. The nitrogen adsorption isotherm and BJH pore distribution( Supporting Information Figure S1) were employed to further discuss the pore structure. For better understanding the electrochemical reaction mechanism of the amorphous ZnO QDs/MPCBs anodes, cyclic voltammetry and galvanostatic charge/discharge test were employed. As shown in the Figure 7(b), the initial capacity is 1914 mAh g-1 (at current density of 100 mA g-1) and charge capacity is 1054 mAh g-1, respectively. Differ from other research, there is no traditional discharge plateau in the first discharge curve of amorphous ZnO QDs/MPCBs anodes but possess a slow decline curve (from 1 to 0.05 V), which may be attributed to the differences in electrochemical reaction between the quantum-sized amorphous ZnO and crystalline ZnO. The initial loss of capacity may be mainly ascribed to the formation of the SEI layer, which was well corresponded to the disappearance of peak and significant difference between the first and subsequent cycles in cyclic voltammetry curve (Figure 7(a)). In the first cathodic cycle, the peak at 1.5 V is related to decomposition of electrolyte and formation of solid electrolyte interphase (SEI) layer, and disappeared in the subsequent cycles. Other peaks at 0.3-0.7 V can be attributed to the reduction of ZnO and Zn-Li alloying process. Finally, it forms a product with a homogeneous distribution of Zn-Li alloy 6


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nanoparticles embedded in Li2O matrix after fully discharged. In the subsequent anodic cycle, a broad peak at around 1.25 V corresponds to the reaction of multistep dealloying process through LiZn→ Li2Zn3→ LiZn2→Li2Zn5→Zn and reformation of ZnO by the redox reaction between Zn and Li2O. After the first cycle, the curves almost overlap in subsequent cycles, indicating good reversibility and stability. The synergy beneficial effect of the features in amorphous ZnO QDs/MPCBs anodes was further identified by using Nyquist plots for amorphous ZnO QDs/MPCBs and the commercial ZnO(20 nm)nanopowder (Supporting Information, Figure S2). According to the Nyquist plots in Supporting Information Figure S2, the lower combined R(SEI+CT) resistance (smaller semi-circular zone)indicating a better electronic conductivity and faster charge transfer of the amorphous ZnO QDs/MPCBs. The sloping straight line in the low frequency zone reflects the diffusion properties of Li+ ions in solid materials (Zw). The improved conductivity give raise to a better dynamic performance. Figure

7(c)

shows

the

cycling

performance

of

amorphous

ZnO

QDs/MPCBs

anodes.

In addition to the loss of the first cycle, there is no observable capacity fading in subsequent 85 cycles. Reversible capacity 930 mAh g-1 (at current density of 100 mA g-1) could be maintained with nearly 90% retention for 85 cycles. Further cycling tests at current density of 200 mA g−1 and 1000 mA g−1 were employed to verify its cycling stability at higher rate and longer cycle. As shown in the Figure 7(d), the amorphous ZnO QDs/MPCBs exhibited a remarkable cycling stability with only approximate 6.9 % capacity fading after 280 cycles at current density of 200 mA g−1, and 6 % after 400 cycles at 1000 mA g−1, respectively. A reversible capacity of 840 mAh g-1 (at 200 mA g−1) and 510 mAh g−1 (at 1000 mA g−1) was also obtained. For studying the rate performance, we change the rate to 50, 100, 200, 500, 1000, 2000, 4000 and 8000 mA g-1 in steps, corresponding reversible capacity of 1110, 980, 860, 700, 530, 340,280 and 210 mAh g-1 (shown in Figure 7(e)) was obtained, respectively. When the rate go back to 100 mA g-1, a capacity of 1050 mAh g-1 was recorded, which was almost 95% of previous capacity obtained at the same rate. When compared to the performance of the recent reports based on crystalline ZnO/C electrodes4, 31, 36-39 (shown in Table 1), the amorphous ZnO QDs/MPCBs showed an excellent cycling stability and long life performance, and delivers a competitive high capacity as well. In order to explain the excellent cycling stability of the the amorphous ZnO QDs/MPCBs anode, HRTEM and Elemental mapping images after 100 charge/discharge cycles(at 500 mA g-1) were provided . As shown in Supporting Information Figure S3, the distribution of zinc oxide still keeps a monodispersed state without significant ZnO agglomeration is found.

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The superior performance may be ascribed to the synergy of the features in amorphous ZnO QDs/MPCBs anodes, including stable interconnected electrical conductive network, small-sized ZnO QDs (smaller than 3 nm), amorphous nature, abundant heterogeneous functional groups, interparticle electrical contact and amorphous carbon encapsulating. On one hand, the small size and amorphous state of primary particles may advance the tolerance of harsh volume change upon Li+ ex/insertion, thus facilitate to maintain the integrity of the electrode; on the other hand, since each amorphous ZnO QDs is encapsulated in an amorphous carbon matrix, therefore, such a dispersion status not only provide a shorter diffusion distance and multidimensional electron transport pathways to increase the rate capacity of the anode material, which is reflected in the rate capability results (Figure 7(e)), but also minimize the stress in grains. Moreover, the unique structure, in which each ZnO QD was separated by amorphous carbon matrix, makes it possess a huge interface between amorphous ZnO QDs and carbon matrix. Thus, it may cause an extra Li+ storage arise from charge separation and interfacial charge storage37, the abundant heterogeneous functional groups exist in carbon matrix may also provide active sites for Li+ storage, to which the extra capacity is attributed. The tolerance of harsh volume change enhanced by nano-sized, amorphization and carbon encapsulating might play a crucial role for the superior reversibility of the electrode. It is worth mentioning that the carbon matrix also provides a very good interparticle electrical contact, and prevents the cracks, pulverization and aggregation of the particles; moreover, it also accommodates the strain of volume change upon consecutive Li+ ex/insertion, further improving the reversibility and integration of the electrode.22 Besides the carbon encapsulating and very small particles, the amorphous state of ZnO QDs is another important factor lead to enhanced performance. In addition to the mentioned virtues of ZnO\C composite mentioned in the part one of the article, there are other advantages for amorphous ZnO QDs/mesoporous carbon bubble composite due to the amorphous state, such as isotropic Li+ storage, superior electrochemical activity and the ability of circumventing the stresses accompanying phase transitions upon Li+ ex/insertion.23

Conclusions In summary, we have developed a facile method to prepare the amorphous ZnO quantum dots (QDs)/mesoporous carbon bubble composite. The composite consists of two parts: mesoporous carbon bubbles as a flexible skeleton and monodisperse zinc oxide QDs (smaller than 3 nm) encapsulated by amorphous carbon matrix as a continuous coating tightly anchored on the surface of mesoporous carbon 8


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bubbles. Thus, the hybrid has both the amorphous QDs feature of high electrochemical activity and the carbon framework abilities of both buffering the volumetric variation to avoid pulverization of electrode and providing stable electron/ion channels to improve the electrochemical performance. It delivers a high reversible capacity of nearly 930 mA h g-1 (at current density of 100 mA g-1) with more than 90% retention for 85 cycles. We believe that this work brings a new horizon in designing high-performance metal oxide electrode materials for LIBs.

Experimental Section Synthesis of mesoporous carbon bubble The zinc oxide nanoparticles with mesoporous carbon bubble were synthesised by a simple one-step direct templating physical vapor deposition method32 which is demonstrated as follow diagrammatic sketch in Fig 1. In the first process, 120 mg commercially available glucose was used as carbon sources and 5 g micrometer-sized zinc powders (Shanghai Jingchun Reagent Co. China, 98%) as the template. Firstly, zinc powders were impregnated by 2 mL 0.005 M glucose ethylene glycol solvent. With ultrasonic oscillation processing, Zn powders and glucose were well dispersion, after this, the mixed slurry was heated in an electric oven at 300 °C for 2 min to dry out ethylene glycol. Then, Zn/C were heated to 780 °C and hold for 20 min, note that there is a plateau at 350 °C for 30 min, in a tubular furnace under vacuum circumstances. Then, zinc powders would be evaporated and escaped when near 410 °C. Finally, with the Zinc consumed completely, the mesoporous carbon bubbles will be obtained. Synthesis of amorphous ZnO/carbon bubble composite The schematic diagram for the synthesis of amorphous ZnO QDs/MPCBs composite is showed in Figure 1. Firstly, 20 mg prepared mesoporous carbon bubbles were fully dispersed in 3 mL ethylene glycol solution containing 120 mg glucose and 140 mg of zinc acetate with ultrasonic treatment, and then annealing in an electric oven at 300 °C for 2 min to dry out ethylene glycol. The precursor was heated to 350 °C and hold for 30 min in a tubular furnace under vacuum circumstances. Almost 80 mg samples can be made at a time. Characterization. Morphologies of the samples were characterized by scanning electron microscopy (SEM,

FEI/Philips XL30). Transmission electron microscopy (TEM) attached with a FEI Tecnai G2 F30 microscope at an operating voltage of 300 kV. Powder X-ray diffraction (XRD) patterns were performed by 9



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XRD (D/MAX 2200 VPC) with Cu Kα (λ= 1.54178 Å) radiation at a generator voltage of 40 kV and a generator current of 40 mA and scanning at 10° min-1 from 20° to 80°. In order to analyze the chemical compositions of the products, XPS measurements were employed with two separate systems equipped with monochromatic Al K sources (ESCALab 250, USA). The Thermogravimetry analyzer (TG-209) was employed to investigat content of carbon of amorphous ZnO QDs/MPCBs with the heating rate of 10 °C /min from 20 °C to 800 °C in air. For the electrochemical tests, the working electrodes were made by uniformly coating the slurry of active materials (ZnO QDs/mesoporous carbon bubble composite), acetylene black, and polytetrafluoroethylene (PTFE) with a weight ratio of 8:1:1 onto a copper foil, and drying at 90 °C for 12 h in air. After that, the foil was tailored to an appropriate size by a coin-type cell microtome (T-06). The coin cells were then assembled by using the ZnO QDs/mesoporous carbon bubble composite as the working electrode, lithium metal foil as the counter electrode, a separator (Celgard, 2400, USA) ,and the electrolyte of 1 M LiPF6 in a 1:1 ethylene carbonate and diethyl carbonate mixture in an argon-filled glove-box. Cyclic voltammograms of the composite electrodes was performed in a voltage window of 0.005–3 V vs. Li+/Li at a scan rate of 0.2 mV s-1 on an Ivium electrochemical workstation. For cycling and rate performance, The galvanostatic cycling experiments were tested at various current densities in a voltage range of 0.01–3.00 V (vs. Li+/Li) at room temperature on a multichannel Neware battery testing system.

ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (11274392, U1401241).

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18. Wang, H. B.; Pan, Q. M.; Cheng, Y. X.; Zhao, J. W.; Yin, G. P., Evaluation of ZnO Nanorod Arrays with Dandelion-Like Morphology as Negative Electrodes for Lithium-Ion Batteries. Electrochim. Acta 2009, 54 (10), 2851-2855. 19. Shen, L. S.; Wang, C. X., ZnO/C Microboxes Derived from Coordination Polymer Particles for Superior Lithium Ion Battery Anodes. RSC Adv. 2015, 5 (108), 88989-88995. 20. Li, P.; Liu, Y.; Liu, J. Y.; Li, Z. T.; Wu, G. L.; Wu, M. B., Facile Synthesis of ZnO/Mesoporous Carbon Nanocomposites as High-performance Anode for Lithium-Ion Battery. Chem. Eng. J. 2015, 271, 173-179. 21. Yang, G. Z.; Song, H. W.; Cui, H.; Liu, Y. C.; Wang, C. X., Ultrafast Li-Ion Battery Anode with Superlong Life and Excellent Cycling Stability from Strongly Coupled ZnO Nanoparticle/Conductive Nanocarbon Skeleton Hybrid Materials. Nano Energy 2013, 2 (5), 579-585. 22. Li, H. Q.; Zhou, H. S., Enhancing The Performances of Li-Ion Batteries by Carbon-Coating: Present and Future. Chemical Communications 2012, 48 (9), 1201-1217. 23. Wang, X. L.; Han, W. Q.; Chen, H. Y.; Bai, J. M.; Tyson, T. A.; Yu, X. Q.; Wang, X. J.; Yang, X. Q., Amorphous Hierarchical Porous GeOx as High-Capacity Anodes for Li Ion Batteries with Very Long Cycling Life. J. Am. Chem. Soc. 2011, 133 (51), 20692-20695. 24. Cui, L. F.; Ruffo, R.; Chan, C. K.; Peng, H. L.; Cui, Y., Crystalline-Amorphous Core-Shell Silicon Nanowires for High Capacity and High Current Battery Electrodes. Nano Lett. 2009, 9 (1), 491-495. 25. Fan, Q.; Chupas, P. J.; Whittingham, M. S., Characterization of Amorphous and Crystalline Tin-Cobalt Anodes. Electrochem. Solid State Lett. 2007, 10 (12), A274-A278. 26. Fang, L.; Chowdari, B. V. R., Sn-Ca Amorphous Alloy as Anode for Lithium Ion Battery. J. Power Sources (Netherlands) 2001, 97-8, 181-184. 13



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27. Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T., Tin-based Amorphous Oxide: A High-Capacity Lithium-Ion Storage Material. Science 1997, 276 (5317), 1395-1397. 28. Li, X. F.; Meng, X. B.; Liu, J.; Geng, D. S.; Zhang, Y.; Banis, M. N.; Li, Y. L.; Yang, J. L.; Li, R. Y.; Sun, X. L.; Cai, M.; Verbrugge, M. W., Tin Oxide with Controlled Morphology and Crystallinity by Atomic Layer Deposition onto Graphene Nanosheets for Enhanced Lithium Storage. Adv. Funct. Mater. 2012, 22 (8), 1647-1654. 29. Han, F.; Li, W. C.; Lei, C.; He, B.; Oshida, K.; Lu, A. H., Selective Formation of Carbon-Coated, Metastable Amorphous ZnSnO3 Nanocubes Containing Mesopores for Use as High-Capacity Lithium-Ion Battery. Small 2014, 10 (13), 2637-2644. 30. Nai, J. W.; Kang, J. X.; Guo, L., Tailoring The Shape of Amorphous Nanomaterials: Recent Developments and Applications. Sci. China-Mater. 2015, 58 (1), 44-59. 31. Zhang, G. H.; Zhang, H.; Zhang, X.; Zeng, W.; Su, Q. M.; Du, G. H.; Duan, H. G., Solid-Solution-Like ZnO/C Composites as Excellent Anode Materials for Lithium Ion Batteries. Electrochim. Acta 2015, 186, 165-173. 32. Song, H. W.; Yang, G. Z.; Wang, C. X., General Scalable Strategy toward Heterogeneously Doped Hierarchical Porous Graphitic Carbon Bubbles for Lithium-Ion Battery Anodes. ACS Appl. Mater. Interfaces 2014, 6 (23), 21661-21668. 33. Song, H. W.; Li, N.; Cui, H.; Wang, C. X., Enhanced Storage Capability and Kinetic Processes by Poresand Hetero-Atoms- Riched Carbon Nanobubbles for Lithium-Ion and Sodium-Ion Batteries Anodes. Nano Energy 2014, 4, 81-87.

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34. Byon, H. R.; Gallant, B. M.; Lee, S. W.; Shao-Horn, Y., Role of Oxygen Functional Groups in Carbon Nanotube/Graphene Freestanding Electrodes for High Performance Lithium Batteries. Adv. Funct. Mater. 2013, 23 (8), 1037-1045. 35. Kundu, S.; Wang, Y. M.; Xia, W.; Muhler, M., Thermal Stability and Reducibility of Oxygen-Containing Functional Groups on Multiwalled Carbon Nanotube Surfaces: A Quantitative High-Resolution XPS and TPD/TPR Study. J. Phys. Chem. C 2008, 112 (43), 16869-16878. 36. Shen, X. Y.; Mu, D. B.; Chen, S.; Huang, R.; Wu, F., Electrospun Composite of ZnO/Cu Nanocrystals-Implanted Carbon Fibers as An Anode Material with High Rate Capability for Lithium Ion Batteries. J. Mater. Chem. A 2014, 2 (12), 4309-4315. 37. Yang, S. J.; Nam, S.; Kim, T.; Im, J. H.; Jung, H.; Kang, J. H.; Wi, S.; Park, B.; Park, C. R., Preparation and Exceptional Lithium Anodic Performance of Porous Carbon-Coated ZnO Quantum Dots Derived from a Metal-Organic Framework. J. Am. Chem. Soc. 2013, 135 (20), 7394-7397. 38. Kose, H.; Karaal, S.; Aydin, A. O.; Akbulut, H., A Facile Synthesis of Zinc Oxide/Multiwalled Carbon Nanotube Nanocomposite Lithium Ion Battery Anodes by Sol-Gel Method. J. Power Sources (Netherlands) 2015, 295, 235-245. 39. Zhang, G. H.; Hou, S. C.; Zhang, H.; Zeng, W.; Yan, F. L.; Li, C. C.; Duan, H. G., High-Performance and Ultra-Stable Lithium-Ion Batteries Based on MOF-Derived ZnO@ZnO Quantum Dots/C Core-Shell Nanorod Arrays on a Carbon Cloth Anode. Adv. Mater. 2015, 27 (14), 2400-2405.

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Figure captions :

Figure 1. The schematic diagram of the formation of the amorphous ZnO QDs/MPCBs.

Figure 2. (a) X-ray diffraction (XRD) patterns. (b) FT-IR spectra. (c), (d) XPS spectra with ZnLMM and Zn 2p of the amorphous ZnO QDs/MPCBs.

Figure 3. Morphology and structure characterized by SEM. (a) Typical SEM images of the pristine mesoporous carbon bubbles. (b) SEM image of the amorphous ZnO QDs/MPCBs.

Figure 4. Morphology and structure characterized by TEM. (a, c) TEM images, and (d) HRTEM image of the amorphous ZnO QDs/MPCBs. (b) corresponding EDS spectra at the site of (c). The inset in (d) show corresponding SAED pattern.

Figure 5. STEM and element mapping images of the amorphous ZnO QDs/MPCBs. (A) STEM image, (B) Carbon, (C) Oxide, (D) Zinc element mapping, (E) high-magnification STEM and corresponding (F) Zinc, (G) Oxide, (H) Carbon element mapping

Figure 6. Thermogravimetric analysis (TGA) of the amorphous ZnO QDs/MPCBs.

16


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Figure 7. (a) CV curves, and (b) charge/discharge profiles of the amorphous ZnO QDs/MPCBs. Electrochemical performances of (c) long cycle performance at current density of 100 mA g-1, (d) at 200 and 1000 mA g-1, (e) rate performance of the amorphous ZnO QDs/MPCBs.

Table 1. Comparison of performance of amorphous ZnO QDs/MPCBs electode with with those of reported crystal ZnO/C composites.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Carbon coating

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Mesoporous CBs Amorphous ZnO/MPCBs

Zn Powder Amorphous ZnO QDs

amorphous carbon matrix

Figure 1

18


a

20

30

40

50

60

70

80

2 Theta (degree)

c

amorphous ZnO QDs/MPCBs ZnO

pristine MPCBs

500

1000 1500 2000 2500 3000 3500 4000

Wave Number(cm-1)

d

ZnO-986.6 eV

Zn 2p3/2 Intensity (a.u.)

Intensity (a.u.) 980

985

990

995

O-H

10

b

C=C C-OH

Intensity

amorphous ZnO QDs/MPCBs

C-O-C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Transmissivity(%)

Page 19 of 26

1000

Zn 2p1/2 23.1 eV

1020

Binding Energy (eV)

1030

1040

Binding Energy(eV)

Figure 2

19


1050


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b)

Figure 3

20


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Figure 4

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

b

(F)

(E)

Zn (H) O 10 nm

(G) C

Figure 5

22


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Page 23 of 26

H2O

100

Mass (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Thermal decomposition of Functional groups

80 50 %

content of Carbon

60

40 0

200

400

600

Temp./degrees centigrade

Figure 6

23


800


(b) 0.1

3.0

0.0

2.5

Voltage (V)

-0.1 1 st cycle 2 nd 3 rd 4 th 5 th

-0.2 -0.3 -0.4 0.0

0.5

1.0

1.5

2.0

2.5

2.0 1.5 1.0 0.5 0.0 0

3.0

500

1000

1500

2000

Capacity (mAh g-1)

Potential (V)

2500

Capacity (mAh/g)

(c)

1 st cycle 2 nd 3 rd 10 th 50 th

100

2000 1500

100 mA g-1

1000

50

500 0 0

(d)

10

20

30

40

50

60

70

80

Efficiency (%)

Current Density (mA)

(a)

0

Cycle Number

(e)

1500 200 mA g-1

1000

50 1000 mA g-1

500 0

0

50

100

150

200

250

300

350

Cycle Number

2500

100

2000 -1 50 mA g -1 100 mA g -1 200 mA g -1 500 mA g -1 1Ag

1500 1000 500 0

0 400

Efficiency (%)

100

2000

-1 100 mA g -1 2Ag

50

-1 4 A g 8 A g-1

0 0

50

100

Cycle Number

Figure 7 24


Efficiency (%)

Capacity (mAh/g)

2500

Capacity (mAh/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Sample name


Reversible Capacity

Current density

(mAh g-1)

(mA g-1)

Cycle number

Capacity retention

930 mAh g-1

100 mA g-1

85th

90 %

840 mAh g-1

200 mA g-1

280th

93.1 %

510 mAh g-1

1000 mA g-1

400th

94 %

809 mAh g-1

80 mA g-1

100th

93 %

450 mAh g-1

350 mA g-1

250th

87.7 %

812 mAh g-1

100 mA g-1

50th

~88.5%

618 mAh g-1

500 mA g-1

50th

~

1150 mAh g-1

75 mA g-1

50th

67.6 %

ZnO-MWCNT-GLY38

460 mAh g-1

0.2 C

100th

54 %

ZnO@ZnO QDs/C NRAs39

699 mAh g-1

500 mA g-1

100th

98 %

SSL ZnO/C NFs 31

814.3 mAh g−1

100 mA g-1

100th

80 %

Amorphous ZnO QDs/MPCBs

ZnO-VAGNs4

ZnO/Cu/CNFs 36

ZnO QDs@porous carbon (550N)37

Table 1

25



Mesoprous CBs Zn Powder

Amorphous ZnO/MPCBs

Carbon coating Amorphous ZnO QDs

amorphous carbon matrix

2500 100

2000 1500

100 mA g-1

1000

50

500 0 0

10

20

30

40

50

60

Cycle Number

Graphic TOC

26


70

80

0

Efficiency (%)

Capacity (mAh/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Mesoporous ... - ACS Publications

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