Fibrous-Root-Inspired Design and Lithium Storage Applications of a

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A Fibrous-Root-Inspired Design and Lithium Storage Applications of a Co-Zn Binary Synergistic Nanoarray System Jia Yu, Shimou Chen, Wenjun Hao, and Suojiang Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b07352 • Publication Date (Web): 03 Feb 2016 Downloaded from http://pubs.acs.org on February 5, 2016

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A Fibrous-Root-Inspired Design and Lithium Storage Applications of a Co-Zn Binary Synergistic Nanoarray System Jia Yuψ,ξ, Shimou Chenξ,*, Wenjun Haoψ,ξ, and Suojiang Zhangξ,* ψ

Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese

Academy of Sciences, Beijing 100190, P. R. China, and ξUniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China. KEYWORDS: lithium ion battery, binary nanoarray, hierarchical structure, synergistic system, biomimetic material, transition metal oxide

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ABSTRACT: Developing lithium ion batteries (LIBs) with fast charging/discharging capability and high capacity is a significant issue for future technical requirements. Transition metal oxides (TMOs) materials are widely studied as the next-generation LIB anode to satisfy this requirement, due to their specific capacity nearly three times than that of conventional graphite anode and low cost. Meanwhile, they also suffer from slow lithium diffusion and limited electrochemical and structural stability, especially at high charging/discharging rate. The structure design of TMO is an effective strategy to obtain desirable LIB performance. Herein, inspired by natural fibrous roots consisting of functional and supporting units that can enhance substances and energy exchange efficiently, fibrous-root-like ZnxCo3-xO4@Zn1-yCoyO binary TMO nanoarrays are designed and synthesized on Cu substrates through a facile one-pot, successive-deposition process, for use as an integrated LIB anode. In a multilevel array ordered by orientation, ultrafine ZnxCo3-xO4 nanowire functional units and stable Zn1-yCoyO nanorod supporting units synergize, resulting in superior rate performance. At a high current density of 500 mAg-1, they could maintain a discharge capacity as high as 804 mAhg-1 after 100 cycles, working much higher than unary cobalt-based and zinc-based nanoarrays. This binary synergistic nanoarray system identifies an optimized electrode design strategy for advanced battery materials.


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Natural hierarchical structures usually exhibit exceptional properties because of the synergy between their multi components, and building similar structures in materials to improve performance has remained a major frontier issue.1-5 For instance, the micro-papillaes of lotus leaves can enhance hydrophobicity while nano-branches provide the critical low sliding angle, inspiring Jiang et al. to construct an artificial super-hydrophobic surface.3 Similarly, Yang et al. fabricated semiconductor nanowire integrated systems for solar water splitting and Dai et al. prepared carbon nanotube arrays with strong binding by imitating natural photosynthetic systems and gecko feet, respectively.4,5 Therefore, 3D hierarchical structures with large special surface, better permeability and more active sites have great potential in electrochemical, optical and catalytic fields.6-8 Herein, fibrous roots have attracted our attention since they elaborately contain a binary synergistic system that consists of primary supporting units (root part) and secondary functional units (fibrous part), to provide efficient absorption and stable support for plants. Compared with most of the hierarchical heterostructures already applied in biomimetic fields, the fibrous-root structure exhibits ultrafine and ordered secondary units with uniform orientation, thus provides obvious advantages that simultaneously satisfy the demands for functional efficiency and structural stability. With the increasing popularity of electric vehicles and portable electronics, the storage and deliverance of electrochemical energy at high charge/discharge rate is a rapid-growing demand.9,10 Currently plenty of attention has been focused on lithium ion battery (LIBs) electrodes with better rate performance. The intrinsic poor capacity (~370 mAhg-1) of commercial graphite anodes has impeded their further applications. Despite holding a superior theoretical capacity, the Si anode face drastic capacity fade at high rate conditions, because of its


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inevasible huge volume change and low conductivity.11 Therefore, with a considerable special capacity (~1000 mAhg-1) and electrochemical and structural stability, transition metal oxides (TMOs) have attracted extensive interests as next-generation LIB anodes for rapid charging and discharging.12-15 To obtain desirable application performance, structure optimization was usually an effective strategy: increasing the specific surface area to enhance the rate capacity, and maintaining a large free volume as a buffer for expansion. Consequently, reports have revealed self-supporting arrays as a promising solution.16-19 However, for traditional TMO arrays, an ordinary diameter will suffer from kinetic problems of Li+ diffusion.20 Thus, to simultaneously realizing high charge/discharge rate and long-term Li storage, generating TMO arrays with stable ordered orientations and diameters as small as tens of nanometers through simple processes remains a challenge.16,18,20 Due to the ease of morphological and compositional adjustability during hydrothermal synthesis of TMOs,21,22 we proposed a fibrous-root-mimetic design and a one-pot synthesis process that utilizes the solubility product (Ksp) gradient in a fluorine-ion-mediated system, to construct desired LIB anodes. Ultrafine nanowire functional units and stable nanorod supporting units were integrated into a fibrous-root-like hetero-hierarchical nanoarray, producing a binary synergistic system consisting of both functional and supporting layers. Compared with traditional one-dimensional TMO nanoarrays, the binary system contained finer array units which improved efficiency and thus high rate capability; compared with ordinary TMO hierarchical structures, it exhibited a more ordered orientation overall, suggesting good durability. Therefore, this biomimetic system design will exhibit great potential for wider systems that exchange substances and energy. For example, we propose that by utilizing appropriate binary complementary pairs


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and adjusting the functional/supporting layer height, this binary system might be efficiently applied in upconversion materials for solar cells.23 In this work, we successfully synthesized fibrous-root-like Zn-doped-Co3O4@Co-dopedZnO (ZnxCo3-xO4@Zn1-yCoyO) binary nanoarrays on Cu substrates, via a one-pot, successivedeposition process in a fluorine-mediated hydrothermal system. Choosing the optimal building units was key: On one hand, zinc-based oxide nanorods were the primary structural materials used as supporting units, because of their high chemical stability, conductivity and considerable Li-storage capacity.24 More importantly, compared with other TMOs, zinc-based hydroxide was easier to fabricate into arrays via a hydrothermal system.25,26 Therefore, zinc-based oxide nanorods could serve as excellent substrates for growing an ultrafine secondary structure, guaranteeing multilevel orientational ordering of the binary system. One the other hand, cobaltbased oxide nanowires were the secondary structural materials serving as functional units because of their superior LIB anode performance, that guaranteed a high efficiency for the binary system.19,27,28 Recently, assembling two different TMOs into a bi-component nanohybrid, particularly with nanoscale homogenerous dispersion has attracted plenty of attention, for better electrochemical performance than that of each component as an LIB electrode. Some bicomponent architectures have been examined, which all exhibited superior Li-storage performance.18, 29-35 In previous reports, most TMO hetero-hierarchically ordered nanostructures were prepared using multi-step methods, rendering our process comparatively facile and efficient. 22,36--38


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RESULTS AND DISCUSSION Synthetic Procedure. The detailed synthetic procedure is described in Scheme 1. First, a specific pretreatment on Cu substrates was required for the one-pot synthesis. Fresh Cu mesh was easily coated with a surface oxide layer that tended to carry positive charges in this hydrothermal system, thus preventing ZnF+ from nucleating first.39,40 After a precisely controlled acid and ultrasonic pretreatment, the oxide layer was partially etched and a nanoscale island-like surface morphology was formed because of the freshly exposed Cu (Scheme 1a and 1b). This “nano-island” active surface served as primary growth points for the binary nanoarrays. During the initial hydrothermal stage, Zn1-yCoyO (0 < y < 0.2) precursor nanorod arrays were grown in situ from the primary growth points, forming a prism structure (Scheme 1c). After the primary growth stage was complete, numerous tiny protrusions appeared on the top platforms of the nanorods rather than on their sides (Scheme 1d). These protrusions served as secondary growth points. Subsequently, ultrafine ZnxCo3-xO4 (0 < x < 1) precursor nanowire arrays grew from protrusions in directions nearly parallel to the primary nanorods (Scheme 1e). After the growth of the secondary nanowire structure had completed, the fibrous-root-structure ZnxCo3-xO4@Zn1yCoyO

precursor nanoarrays were prepared, while the primary and secondary structures

corresponded to the root and fibrous parts respectively. After calcination, the fibrous-root structure was retained well for the ZnxCo3-xO4@Zn1-yCoyO binary nanoarrays (Scheme 1f).


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Scheme 1. The preparation procedure: a) The original copper mesh coated with oxide layer. b) The pretreated copper mesh with “nano-island” active surface as primary growing points. c) Prism structure Zn1-yCoyO precursor nanorod arrays. d) ZnxCo3-xO4 precursor protrusions appeared as secondary growing points. e) Growing fibrous-root structure precursor nanoarrays. (f) Fibrous-root structure ZnxCo3-xO4@Zn1-yCoyO binary nanoarrays . Morphology and Structural Characterization. Products morphologies at various stages were investigated by scanning electron microscope (SEM) and high-resolution SEM (HRSEM) (Figure 1). As shown in Figure 1a, the original Cu mesh exhibited a relatively smooth surface. By contrast, the pretreated Cu mesh was accompanied by uniform etched rhombus, showing a unique “nano-island” morphology which was of fine parallelism with hydrothermal products (Figure 1b). The duration of the ultrasonic treatment was key, too short pretreatment time could not guarantee the successful formation of the island-like structure, without which undesirable irregular particles would be the main hydrothermal products instead of nanoarrays (Figure S1a and S1b, Supporting Information). After 3 h of in situ growing on Cu substrates, Zn1-yCoyO


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precursor nanorod arrays with rhombic cross-sections were fabricated, with lengths of 2–4 µm and diameters of 0.7–1.2 µm (Figure 1c and Figure S2a). Mainly locating at the top platforms of nanorods, protrusions were clearly observed after 4 h of hydrothermal time, when the primary growth stage was complete (Figure 1d and Figure S2b). Figure 1e and Figure S2c showed the ultrafine ZnxCo3-xO4 precursor nanowire arrays with diameters of only 30~40 nm, which grew in directions nearly parallel to the primary nanorods, to form a growing fibrous-root-structure. After 10 h of hydrothermal time, the average length of the ZnxCo3-xO4 precursor nanowires exceeded 5µm, to form the final fibrous-root-structure (Figure 1f and Figure S2d, S3). After calcination, a fibrous-root-like binary system consisting of ZnxCo3-xO4 functional layer and Zn1-yCoyO supporting layer was obtained (Figure S4).

Figure 1. SEM images of surface morphologies of a) un-pretreated Cu mesh and b) pretreated Cu mesh with precisely-controlled HCl and ultrasonic treatment (1 h); product morphologies during various hydrothermal stages: c) By 3 h, Zn1-yCoyO precursor nanorod arrays (prism


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structure). d) By 4 h, ZnxCo3-xO4 precursors protrusions appeared (prism structure with protrusions). e) By 6 h, growing ZnxCo3-xO4@Zn1-yCoyO precursor nanoarrays (growing fibrousroot structure). f) By 10 h, final ZnxCo3-xO4@Zn1-yCoyO precursor nanoarrays (fibrous-root structure). Insets of c-e: Corresponding HRSEM images. The fine structures were observed by transmission electron microscopy (TEM) and highresolution TEM (HRTEM). Because the final fibrous-root structures with long fibrous parts were easily damaged during ultrasonic dispersion, growing fibrous-root structures whose fibrous parts were comparatively shorter were observed by TEM and characterized by energy dispersive spectrometry (EDS) analysis. Figure 2a clearly showed the fibrous-root-like morphology, and each part of this binary system could be distinguished. At the heterojunction, it was observed that the ZnxCo3-xO4 nanowires have their roots inside the Zn1-yCoyO nanorods, suggesting that the ZnxCo3-xO4 nanorods are not just loosely attached to the nanorod surface (Figure S5a). Figure 2b and Figure S5b showed the polycrystalline nature of the fibrous ZnxCo3-xO4 nanowire region, which was further proved by the selected area electron diffraction (SAED) pattern (Inset of Figure S5b). Moreover, from the HRTEM image, a porous structure was clearly observed, due to the gas releasing during calcination process. Porous ultrafine nanowires might provide better access for Li+ while shortening diffusion paths, thereby improving the rate capability.9d In addition, the nitrogen adsorption-desorption isotherm and pore size distribution curve of the integrate electrode loading ZnxCo3-xO4@Zn1-yCoyO binary nanoarrays were provided (Table S1, Supporting Information). In addition, from TEM observations, it was suggested that the phase boundary between ZnxCo3-xO4 and Zn1-yCoyO might enable extra interfacial lithium storage, which would be benefit for the special capacity of system.41 On the basis of the HRTEM observations (Figure 2c and Figure S5c), the dominant 0.468 nm interplanar spacing of the


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nanowires is consistent with the spacing of the (111) facets of spinel-phase Co3O4, which, as a LIB anode material, exhibits superior capacity and rate performance compared to those of the usual (001)-faceted Co3O4.42,43 In addition, (220) lattices were also observed. The preferential formation of the (111) orientation might be related to the secondary growth mechanism.

Figure 2. TEM images of a) growing fibrous-root structure precursor and b) ZnxCo3-xO4 nanowire fibrous part with a porous and polycrystalline structure. c) HRTEM image of ZnxCo3xO4

nanowires with dominantly exposed (111) facets and a little (220) facets; XRD patterns of


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the d) precursor and e) final product of ZnxCo3-xO4@Zn1-yCoyO binary nanoarrays; f) EDS mappings and g,h) line-scan analysis of the fibrous-root structure, demonstrating a Zn-rich root part and a Co-rich fibrous part, while each part showing decreasing Zn concentration and increasing Co concentration towards growth direction. Component Characterization. The Co2+ partially replaced Zn2+ in the ZnO through an isomorphous substitution to form Zn1-yCoyO,44 and Zn2+ was embedded in the Co3O4 lattice by partially replacing the original Co2+ to form ZnxCo3-xO4.13 These two phase composed the root and fibrous parts of the binary nanoarrays with fibrous-root structure, respectively. The crystalline structures of the prepared nanoarrays were investigated by X-ray diffraction (XRD) (Figure 2e), diffraction peaks were observed for both hexagonal wurtzite ZnO (JCPDS card no. 36-1451, space group: P63mc) and cubic spinel Co3O4 (JCPDS card no. 42-1467, space group: Fd3m).45,46 A small quantity of doping had little effect on the lattice of original pure phase, while efficiently improving the rate performance and stability as LIB anodes.47,48 X-ray photoelectron spectroscopy (XPS) was also used to investigate chemical compositions. For O1s states, two peaks at approximately 531 eV and 530 eV were attributed to the ZnO and Co3O4, respectively (Figure S6). For this fibrous-root structure, Zn-rich/Co-poor root parts and Co-rich/Zn-poor fibrous parts were clearly revealed using EDS elemental mappings (Figure 2f and Figure S7). Moreover, EDS line-scan analyses in the axial (Figure 2g and 2h) and radial directions (Table S2) indicated another significant compositional character: Root parts and fibrous parts each exhibited decreasing Zn concentrations and increasing Co concentrations in the growth direction, which was also supported by electron probe X-ray microanalysis (EPMA) (Table S3). This concentration gradient is interestingly similar to that of natural fibrous roots.49 Either integral


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Co-Zn combination or partial Co-Zn gradient, each was able to efficiently integrate electrochemical performance advantages, and this combination-transition design idea had been proved to greatly enhance system performance and stability.8,50 Synthetic Mechanism. The proposed synthesis mechanism for this Co-Zn binary system was as follows.51-53 And the introduction of fluorine ions in the hydrothermal system should be the prerequisite for achieving ZnxCo3-xO4@Zn1-yCoyO rather than ZnCo2O4 nanostructures.51,52 On the one hand, the carbonate and hydroxyl anions generated by the hydrolysis of urea have an key effect on the growth of products. On the other hand, in this homogeneous fluorine-mediated system, Co2+ and Zn2+ tended to form CoF+ and ZnF+ complexes. Because of a lower solubility product (Ksp), initially the ZnF+ combined with OH- to precipitate Zn(OH)F nanorod arrays, which were precursors of ZnO. Related chemical reactions were as follows, while Co partially replaced Zn. +

CO(NH2 ) 2 + 3H 2 O → 2NH4 + 2OH− + CO 2

(1)

ZnF + + OH − → Zn(OH)F ↓

(2)

By this stage, primary Zn(OH)F nanorod arrays were successfully in situ grown on Cu mesh substrates. Subsequently, when the ZnF+ was consumed leaving a low concentration, concurrently the CoF+ began to precipitate on the primary Zn(OH)F nanorods. Herein carbonate anions would replace F− owing to its strong affinity to Co2+, thus the Co(CO3)0.5(OH)•0.11H2O nanowire arrays grew as precursors of Co3O4.53,54 Related chemical reactions were as follows, while Zn partially replaced Co.

CO 2 + H 2 O → 2H + + CO 3

2-

2−

CoF+ + OH − + 0.5CO3 + 0.11H 2O → Co(CO3 )0.5 (OH) ⋅ 0.11H2O ↓ + F−

(3) (4)


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The intrinsic Co-Zn concentration-gradient might originate from the preferential precipitation of Zn in solid phase, when compared with the liquid phase reactant system. For the precursor of binary nanoarrays, XRD diffraction peaks for orthorhombic Zn(OH)F (JCPDS card no. 74-1861, space group: Pna21) and orthorhombic Co(CO3)0.5(OH)•0.11H2O (JCPDS card no. 48-0083, space group: P2212) were observed (Figure 2d).51,53 And corresponding EDS elemental mappings were shown in Figure S8. Besides the hydrogen element that could not be detected by EDS, the fibrous part of the hierarchical structure consisted of Co, C and O elements, whereas the root part consisted of Zn, Co and O, indicating that the secondary structure was mainly made of Co(CO3)0.5(OH)•0.11H2O and the primary structure was Zn(OH)F. Electrochemical Investigation. Similar to natural fibrous roots that can greatly improve nutrient absorption while simultaneously providing structural support to plants, this binary synergistic nanoarray system was expected to exhibit considerable advantages on the balance of efficiency and stability as a LIB integrated anode. Therefore, electrochemical performances of the

fibrous-root

structure

ZnxCo3-xO4@Zn1-yCoyO

nanoarrays

as

LIB

anodes

were

comprehensively compared with those of the Zn1-yCoyO nanorod arrays and traditional Co3O4 nanowire arrays. When the Zn source was absent in the hydrothermal system, traditional selfsupported Co3O4 nanowire arrays were obtained under same conditions. But they exhibited a larger diameter of more than 100 nm and a lower orientational ordering than those of fibrousroot-structure nanoarrays (Figure S9). Figure 3a showed their first charge-discharge curves at a current density of 500 mAg-1 and a voltage range of 0.01–3.0 V (vs. Li/Li+). In the case of the diphase ZnxCo3-xO4@Zn1-yCoyO, the two discharge voltage plateaus at approximately 1.2-1.3 V and 0.7 V corresponded to the reduction of ZnxCo3-xO4 and Zn1-yCoyO, respectively.12,24 The initial discharge capacities of the


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ZnxCo3-xO4@Zn1-yCoyO, Zn1-yCoyO and Co3O4 anodes were 1692, 1258 and 1284 mAhg-1 respectively, all of which were higher than the theoretical capacities of cobalt-based and zincbased oxides (ZnO: 978 mAhg-1; Co3O4: 890 mAhg-1; ZnCo2O4: 970 mAhg-1) while ZnxCo3xO4@Zn1-yCoyO

exhibited the highest capacity.13,24,28 Their extra capacities were attributed to the

formation of a solid electrolyte interphase (SEI) film and a series of irreversible reactions involving the electrolyte,55,56 and possible interfacial lithium storage.41 The initial charge capacities of the ZnxCo3-xO4@Zn1-yCoyO, Zn1-yCoyO and Co3O4 anodes were 1241, 789 and 861 mAhg-1, respectively. Their relatively low initial efficiencies mainly resulted from a series of irreversible reactions involving the electrolyte.

Figure 3. a) First charge-discharge curves for fibrous-root structure ZnxCo3-xO4@Zn1-yCoyO nanoarray anode, prism structure Zn1-yCoyO nanorod array anode and Co3O4 nanowire array


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anode, at a current density of 500mAg-1 and a voltage range of 0.01−3.0V (versus Li/Li+). b) Discharge capacities vs cycle numbers profiles of three anodes at a current density of 500mAg-1. c) Discharge curves of three anodes at various current densities. d) Impedance spectra of three anodes. Figure 3b highlighted the superior cycling performance of the binary ZnxCo3-xO4@Zn1yCoyO

anode at a high rate under galvanostatic conditions, when compared with that of other two

unary anodes. At a high current density of 500 mAg-1, the discharge capacity of the ZnxCo3xO4@Zn1-yCoyO

anode became quite stable after about 40 discharge/charge cycles. After 100

cycles, it could maintain a discharge capacity as high as 804 mAhg-1, reaching more than 90% of its theoretical capacity. However, those of Zn1-yCoyO and Co3O4 were only 384 and 463 mAhg-1 respectively, exhibiting an intensified capacity fading. Meanwhile, the coulombic efficiency of the Zn1-yCoyO was relatively lower and more unstable than other two anodes during long cycles (Figure S10). Based on previous reports, under such high rate charging/discharging, this binary nanoarray system indeed exhibited superior capacity with respect to most of other single or multi-component Co-based/Zn-based nanostructures, including traditional self-supported nanoarrays, nanospheres, nanowires and nanoparticles (Table S4).12,13,19,24,31-35,42,46,57-60 Furthermore, the skeleton of fibrous-root structure anodes was well preserved after 100 cycles, further proving its excellent structural stability (Figure 4). In addition, the influence of the substrate on integrated electrode performance was investigated (Figure S11). It was found that the Cu substrate contributed less than 3% to the whole capacity, indicating a tiny effect.


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Figure 4. SEM images of fibrous-root-structure anode after a) 5 and b) 100 cycles respectively; SEM images of prism-structure anodes after c) 5 and d) 100 cycles, respectively. As the rapid charging/discharging performance plays a critical role in practical applications, discharge capacities of three anodes at various rates were compared, for investigating the rate capability of this binary system (Figure 3c). Obviously, capacities of all three anodes decreased monotonically as the current density increased. However the ZnxCo3-xO4@Zn1-yCoyO anode exhibited far better rate capability. Specifically, at a high rate of 1000 mAg-1, it still delivered a discharge capacity of 738 mAhg-1. This means that the discharge or charge process can be finished in about 43 min, while more than twice the capacity of the commercial graphite is maintained. By contrast, at the same rate, the Zn1-yCoyO and Co3O4 anodes maintained capacities of only 410 and 370 mAhg-1 respectively, which were about half of the that of the ZnxCo3xO4@Zn1-yCoyO

anode. When the rate was increased to 1000 mAg-1, it still delivered a discharge

capacity of more than 730 mAhg-1.Finally, when the current density changed to 200 mAg-1, the discharge capacity of the ZnxCo3-xO4@Zn1-yCoyO recovered to more than 940 mAhg-1, revealing considerable capacity retention.


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In order to investigate the electrochemical reactions during lithiation/delithiation process, first five cycles of cyclic voltammetry (CV) curves for the ZnxCo3-xO4@Zn1-yCoyO electrode were carried out, at a scan rate of 0.1 mVs-1 and a voltage range of 0.01–3.0 V (Figure S12). The peaks appeared in the 1st cathodic scan indicated irreversible reactions including the formation of the SEI film.61 From the 2nd cycles, CV curves became similar, revealing good cycling stability. Cathodic peaks at around 0.7 and 1.3 V were due to the reduction process of Zn1-yCoyO and ZnxCo3-xO4 respectively.24,61 Figure 3d showed the impedance spectra of the ZnxCo3-xO4@Zn1yCoyO,

Zn1-yCoyO and Co3O4 anodes, which showed similar characteristics: a depressed

semicircle at the high-medium frequency as well as an inclined line at the low frequency. The inclined lines were ascribed to the lithium diffusion impedance, while the depressed semicircles were attributed to charge impedance.62,63 It indicated that the ZnxCo3-xO4@Zn1-yCoyO anode showed lowest impedance, which was helpful for improving the rate capability. For this binary nanoarray system, the high-performance Zn-doping Co3O4 functional layer and stable Co-doping ZnO supporting layer synergized to take full advantages of each component, successfully achieving superior rate and cycling performance. Discussion. In this study, a binary synergistic nanoarray system (ZnxCo3-xO4@Zn1-yCoyO) performed excellently as an integrated anode for LIBs, especially for the rate performance, when compared with unary cobalt-based or zinc-based nanoarrays. These results fully reflected the strong potential of fibrous-root-mimetic design, which was attributed to the synergistic effects of several factors. On the one hand, from the componential perspective: 1) The ZnxCo3-xO4 nanowire functional units exhibited performance advantages intrinsic to cobalt-based oxides, while their ultrafine and porous structure was more accessible to Li+ because of the larger electrode–electrolyte interface and the presence of shorter diffusion paths for both Li+ ions and


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electrons. 2) The Zn1-yCoyO nanorod supporting units provided a stable skeleton and excellent substrates for the orientationally ordered growth of the ultrafine ZnxCo3-xO4 nanowires. Meanwhile they also played a considerable role in Li storage. 3) The doping effect efficiently enhanced conductivity and increased the cell parameters, expanding the tunnels for Li+ diffusion and stabilizing the cell frame.47 On the other hand, from the structural perspective: 1) The entire orientationally ordered multilevel array provided sufficient free volume to buffer the volume change during Li+ insertion/extraction. 2) The ZnxCo3-xO4/Zn1-yCoyO two-phase boundary enabled extra interfacial lithium storage.


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CONCLUSSIONS In summary, for achieving superior rate performance as LIB anodes, structural design of TMO was adopted as a significant and effective strategy. Inspired by natural fibrous-root structures that provide nutrient absorption and structural support to plants efficiently, a binary synergistic nanoarray system consisting of functional and supporting units was proposed. Fibrous-root-like ZnxCo3-xO4@Zn1-yCoyO nanoarrays were successfully grown on Cu current collectors via a one-pot successive-deposition process, for use as an integrated anode for LIBs. The ultrafine ZnxCo3-xO4 nanowire arrays were primarily grown on top of the platforms on the Zn1-yCoyO nanorod arrays along the parallel direction, forming the functional and supporting layers of this binary synergistic system, respectively. In a multilevel array ordered by orientation, ultrafine ZnxCo3-xO4 nanowire functional units and stable Zn1-yCoyO nanorod supporting units synergized, resulting in superior rate and cycling performance. It could maintain a capacity as high as 804 mAhg-1 at a high rate of 500 mAg-1 after 100 cycles, being far better than unary cobalt-based and zinc-based nanoarrays. Due to its superior performance and efficient one-pot synthesis process, this design has strong potential for wide systems that exchange substances and energy, such as LIB anode materials and enhanced upconversion materials for solar cells.


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EXPERIMENTAL DETAILS Preparation of Materials: All of the hydrothermal chemicals were analytical grade and were used without further purification. Surface Pretreatment of the Cu Substrates: During a typical experiment, a whole piece of Cu mesh (300-mesh, purity over 95%, thickness of 70 µm) was first cut into smaller 5× 3 cm2 pieces. These small pieces were soaked in 10% hydrochloric acid and stirred for 12 h at 25 °C, followed by 1 h (or 30 min as a control group) of ultrasonic treatment at 40 °C. The pretreated Cu mesh pieces were subsequently washed for several times with deionized water and ethanol. Preparation of ZnxCo3-xO4@Zn1-yCoyO and Zn1-yCoyO nanoarrays: First, 2 mmol of Zn(NO3)2•6H2O, 5 mmol of Co(NO3)2•6H2O, 5 mmol of CO(NH2)2 and 2 mmol of NH4F were dissolved in 40 mL of deionized water. After stirring and 10 min of ultrasonic treatment, the mixed solution was transferred to an autoclave, and a piece of pretreated Cu mesh was subsequently immersed. After 10 h of hydrothermal reaction at 180 °C, the obtained Cu mesh that carried products was rinsed with deionized water and ethanol and then dried at 70 °C for 12 h. Finally, the Cu mesh was calcined at 400 °C for 3 h to obtain the final fibrous-root structure ZnxCo3-xO4@Zn1-yCoyO nanoarrays, with a colour changing from pink to black. When the hydrothermal time was reduced to 4 or 7 h, shorter ZnxCo3-xO4 fibrous parts were obtained, as the growing fibrous-root structure nanoarrays. And when the hydrothermal time was further reduced to 3 h, only Zn1-yCoyO nanorod arrays with prism structures were obtained. Preparation of Co3O4 nanowire arrays: The reaction system and conditions were the same as those for preparing the final fibrous-root structure ZnxCo3-xO4@Zn1-yCoyO nanoarrays, except that the Zn(NO3)2•6H2O was omitted.


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Structural and Component Characterization: Scanning electron microscopy (SEM) observations were carried out on a JEOL JSM-7001F microscope operated at 10.0 kV. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) measurements were performed with a JEOL JEM-2100 microscope operated at 200 kV. The specific surface area of the samples was measured at -196 °C with a Quadrasorb SI-MP analyzer using the N2 adsorption method. Before the measurements, samples were degassed under vacuum (4 mmHg) at 150 °C for 7 h. X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Focus X-ray diffractometer equipped with a Ni-filtered Cu-Kα radiation (λ = 0.15406 nm) source, and the 2θ range was between 10° (or 15°) and 70°. The X-ray photoelectron spectroscopy (XPS) data were obtained with a Scienta ESCA-300 using a monochromated AlKα X-ray source (hv = 1486.6 eV) at a power of 2.4 kW and a base pressure of 7.3 × 10-8 Pa in the analytical chamber. The energy dispersive Spectrometer (EDS) elemental mapping and linescan analysis were performed using an Oxford X-MaxN attached to the JEOL JSM-7001F microscope. The EPMA was performed using an X-MaxN silicon drift detector. Electrochemical Measurements: Electrochemical performance measurements were performed at room temperature using CR2032 coin-type half-cells. The working electrodes were a series of Cu meshes carrying active materials (ZnxCo3-xO4@Zn1-yCoyO, Zn1-yCoyO and Co3O4 nanoarrays), and they were first cut into smaller pieces with a diameter of 14 mm prior to testing. To calculate the mass loading of active materials on the Cu mesh substrates, we measured the mass difference between the Cu mesh piece loading active materials and the bare Cu mesh piece. This two kind of Cu mesh pieces were cut into same sizse to ensure the same substrate mass, meanwhile the bare Cu mesh piece was also calcined at same conditions to minimize errors. The lithium foil was used as the counter electrode. The two electrodes were separated using Celgard


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2400 membranes. The electrolyte used in these experiments was LiPF6 (1 M) in EC+DEC+DMC (1:1:1 by weight). To cycle coin-type electrochemical cells, they were discharged with a constant current of 500 mAg−1 to 0.01 V (vs Li/Li+) and charged with a constant current of 500 mAg−1 to 3.0 V (vs Li/Li+). In the rate capability test, the charge/discharge current density was changed every ten cycles according to this sequence: 200, 500, 1000, 1500 and 200 mAg−1. Cyclic voltammetry (CV) data was recorded on an Autolab (PGSTAT302N) electrochemical workstation, with a scanning rate of 0.1 mVs-1 and a voltage range of 0.01-3.0 V. Electrochemical impedance spectroscopy (EIS) measurements were performed using an ACM Gill-AC-4 electrochemical station.


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ASSOCIATED CONTENT Supporting Information. SEM images of pretreated Cu mesh and corresponding products in a control group; supplementary SEM images, TEM images, XPS patterns, EDS mapping patterns, EDS line-scan analysis, EPMA results of the precursors and final products; BET surface area of both structures; coulombic efficiencies of three anodes; performance comparisons with other Cobased/Zn-based nanostructure as LIB anodes; capacity portion of the Cu substrate in the integrated electrode. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Shimou Chen*, Suojiang Zhang* Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China *Address correspondence to: [email protected] ACKNOWLEDGMENT We acknowledge the support of National Natural Science Foundation of China (No. 21276257, 91434203, 91534109) and “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA09010103).


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TABLE OF CONTENTS GRAPHIC AND SYNOPSIS A Co-Zn binary-synergistic nanoarray system is proposed with the inspiration of natural fibrous-root structures. And ZnxCo3-xO4@Zn1-yCoyO nanoarrays are directly synthesized on Cu current collectors through a one-pot, successive-deposition process, for use as an integrated anode for lithium-ion batteries. In an ordered multilevel array, ultrafine Co-based functional units and stable Zn-based supporting units synergize, exhibiting superior electrochemical performance at high rates.


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Fibrous-Root-Inspired Design and Lithium Storage Applications of a

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