reduced graphene oxide with high-performance lithium storage capability

Accepted Manuscript Title: Strongly coupled hybrid ZnCo2 O4 quantum dots/reduced graphene oxide with high-performance lithium storage capability Author: Wei Yao Yi Dai Kang Ge Juhua Luo Qingle Shi Jianguang Xu PII: DOI: Reference:

S0013-4686(16)31319-6 http://dx.doi.org/doi:10.1016/j.electacta.2016.06.002 EA 27441

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

12-4-2016 24-5-2016 1-6-2016

Please cite this article as: Wei Yao, Yi Dai, Kang Ge, Juhua Luo, Qingle Shi, Jianguang Xu, Strongly coupled hybrid ZnCo2O4 quantum dots/reduced graphene oxide with high-performance lithium storage capability, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.06.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Strongly coupled hybrid ZnCo2O4 quantum dots/reduced graphene oxide with high-performance lithium storage capability Wei Yao,* Yi Dai, Kang Ge, Juhua Luo, Qingle Shi, and Jianguang Xu* School of Materials Engineering, Yancheng Institute of Technology, 211 East Jianjun Road, Yancheng, Jiangsu 224051, People’s Republic of China E-mail: [email protected]; [email protected] ABSTRACT: Mesoporous binary metal oxides/reduced graphene oxide (rGO) two-dimensional nanostructures can provide open large surface areas for lithium ion access and storage, holding a great promise as high-performance electrode materials for next-generation energy storage. In this work, we develop an effective strategy involving a simple polyol process and a facile thermal annealing treatment, to synthesize ZnCo2O4 Quantum Dots (QDs)/rGO hybrid. Due to the large specific surface area, strongly coupled interaction and synergic effect between ZnCo2O4 QDs and rGO, the hybrid shows excellent lithium storage ability, with high reversible specific capacity, and superior rate performance, as well as ultralong cycle life. After 100 cycles, the ZnCo2O4 QDs/rGO2 delivers a capacity of 1062 mAh g-1 at a current density of 500 mA g-1. Even cycling at 2000 mA g-1 up to 1000 cycles, the reversible capacity still preserves 682.5 mAh g-1. These electrochemical results indicate the ZnCo2O4 QDs/rGO2 hybrid could be a promising candidate material as a high-performance anode material for lithium-ion batteries. Keywords: ZnCo2O4 quantum dots; reduced graphene oxide; anode materials; synergic effect; Li-ion battery 1

1. Introduction Lithium ion batteries (LIBs) have been intensively applied in the field of electric vehicles, portable electronic devices, and other smart systems, owing to their advantages of high energy density, long lifespan, no memory effect, and environmental benignity [1-4]. With increasing market demands of LIBs used for energy storage, numerous research efforts have focused on the exploration of advanced anode materials with excellent specific capacity and long cycle life [5-6]. Transition metal oxides (TMOs), such as CoOx [7-8], FeOx [9-10] and MnOx [11-12], have delivered much higher theoretical specific capacities (>500 mAh g-1) than that of commercial graphite (372 mAh g-1), through conversion reaction with lithium. Unfortunately, single metal oxides undergo severe volume change during lithium ion insertion/extraction, leading to rapid capacity fading and poor cycling performance [13-14]. Furthermore, the low electrical conductivity of these anodes results in rapidly deterioration of rate capability at high current density [15]. It is interesting to note that binary metal oxides in spinel structure, such as ZnCo2O4 [16-34], NiCo2O4 [35-38], ZnMn2O4 [39-40], ZnFe2O4 [41], CoMn2O4 [42-43], are attracting much attentions as anode materials for LIBs. Among various binary metal oxides above, ZnCo2O4 has been considered attractive in view of its higher electronic conductivity and larger theoretical specific capacity than that of TMOs. More importantly, it could store Li+ through not only the conversion reaction, but also the alloying/dealloying reactions between Zn and Li (Zn + Li+ + e- ↔ LiZn), leading to high theoretical specific capacity of 900 mAh g-1 [44-45]. However, the 2

huge volume changes result in the repeated expansion and contraction of the lattice along with lithiation/delithiation processes, and lead to the poor Coulombic efficiency and undesirable capacity fading of ZnCo2O4 particles [46]. Nanoscale engineering of ZnCo2O4 has been proven to be an effective way to improve its electrochemical Li-storage behavior, as such nano-structure features could reduce volume changes compared with micron-size counterparts over cycling [47-48]. Yet, the rate capacity of ZnCo2O4 nanomaterials is still limited owing to its intrinsically poor electronic/ionic conductivity. To further optimize energy storage performances of anode materials, building appropriate nanostructure of the hybrid with binary metal oxides and carbon is regarded as an appealing strategy. Among various carbon materials, reduced graphene oxide (rGO) may be a superior candidate to make hybrid with spinel ZnCo2O4 as anode materials for LIBs, due to its good electric conductivity, large surface area, and excellent flexibility [49]. Recently, a few ZnCo2O4/rGO, such as ZnCo2O4 nanosheets/rGO [45] and ZnCo2O4 nanoparticles/graphene [44], have been synthesized by in situ growth method or electrostatic adsorption strategy, and demonstrated improved rate capacity in comparison with pure ZnCo2O4. Nevertheless, on account of the weak interaction between ZnCo2O4 and rGO, the performances of hybrids under a high current density (> 1000 mA g-1) still need to be improved. It is believed that strongly coupled interaction between TMOs and rGO could not only increase migration rate of electron and Li ion [12, 15], but also restrain the structure collapse of hybrid upon cycling [50]. On the other hand, the size and dispersion of 3

ZnCo2O4 are also important factors to determine the anode performance. Small particle sizes and good dispersion of ZnCo2O4 quantum dots (QDs) grafted on graphene sheets can endow a large surface area for Li ion transport, benefiting for superior specific capacitance and rate capability [15,51]. Thus, hybrid nanostructure on the basis of ZnCo2O4 QDs and rGO sheets could synergize above advantageous features and achieve further optimization of lithium storage capability. In this work, we report a facile route to prepare ZnCo2O4 QDs/rGO hybrids via a polyol process followed by thermal annealing treatment. Due to good dispersion and small particle sizes of ZnCo2O4 QDs, as well as the intimate interactions between ZnCo2O4 QDs and rGO sheets, the as-prepared ZnCo2O4 QDs/rGO2 delivers excellent lithium storage properties with a high specific capacity of 1062 mA h g-1 after 100 cycles at a current density of 500 mA g-1. Even at a high current density of 2000 mA g-1, capacity retention of 88% (against 2nd capacity) is achieved after 1000 cycles, all of which render it a superior anode material for high performance LIBs. 2. Experimental 2.1. Preparation of ZnCo2O4 QDs/rGO Graphene oxide (GO) was prepared by a modified Hummers method [52]. For the fabrication of ZnCo2O4 QDs/rGO hybrids, 48 mg of GO was dispersed in 120 mL of ethylene glycol dissolved with a certain amount of Zn(Ac)2·2H2O and Co(Ac)2·4H2O in the molar ratio of 1:2. After refluxing at 170 oC for 2 h, the precipitates were collected by centrifugation and washed with distilled water and ethanol three times. Finally, to obtain ZnCo2O4 QDs/rGO hybrids, the precipitates 4

were annealed at 250 oC for 2 h in air with a slow heating rate of 0.5 oC min-1. Different weight ratios of ZnCo2O4 in ZnCo2O4/rGO were explored to attain optimal electrochemical performance. The obtained products were noted as ZnCo2O4 QDs/rGO1, ZnCo2O4 QDs/rGO2, and ZnCo2O4 QDs/rGO3. The corresponding ZnCo2O4 contents in the hybrids were 73.6%, 85.0% and 91.1%. For comparison, pure ZnCo2O4 was prepared by the same method except at an annealing temperature of 400 oC. 2.2. Characterizations Scanning electron microscopy (SEM) images and EDS mapping were recorded on a field emission scanning electron microscopy (FESEM, Hitachi, S4800, Japan) with an accelerating voltage of 10 kV. The morphologies and fine structures were observed by transmission electron microscopy (TEM, JEOL, JEM-2100, Japan). The phase structure was characterized by X-ray diffraction analysis (XRD, Rigaku, D/MAX2500 V, Japan) with Cu-Kα radiation (λ = 1.5418 Å) at room temperature in the 2θ range of 10° to 70°. X-ray photoelectron spectroscopy (XPS) data were obtained using monochromatic Mg-Kα X-rays at hυ = 1253.6 eV. Nitrogen sorption isotherms were measured on the Surface Area and Porosity Analyzer (ASAP 2020). Thermogravimetric analyses (TGA) were performed under air using Pyris1 TGA instrument from room temperature to 700 °C at a heating rate of 10 °C min-1. Raman spectra were recorded on an Invia Raman microscope (Renishaw, UK) with excitation laser beam wavelength of 532 nm. 2.3. Electrochemical Measurements 5

To prepare the working electrode, the slurry was made by mixing the active material, acetylene black and Polyvinylidene Fluoride (PVDF) dissolved in N-methyl-2-pyrrolidinone (NMP) in the weight ratio of 70:20:10. Then the slurry was pasted onto a copper foil and dried in a vacuum oven at 120 oC for 12 h. The average mass loading of all electrodes was about 0.5-1 mg cm-2. Lithium metal was used as the counter electrode and reference electrode. The electrolyte was composed of 1 M LiPF6 in a mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate (1:1:1 in volume). All cells were constructed and handled in an Ar-filled glove box. The galvanostatic charge/discharge measurements were carried out in the voltage range of 0.01-3.0 V on a NEWARE battery tester. Cyclic voltammetry (CV) curves were recorded on a CHI660D electrochemical workstation (Shanghai CH Instrument Company, China) at a scan rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) experiments were tested in the frequency range from 10 mHz to 100 KHz. 3. Results and Discussion The proposed procedure for the preparation of the ZnCo2O4 QDs/rGO is illustrated in Figure 1. In the first step, rich oxygen functional groups (hydroxyl and carboxy groups) on GO strongly couple with the metal ions. During the refluxing process, ZnCo-glycolate QDs can in situ heterogeneously nucleate and grow on the surface of GO sheets derived from the coordination effect [53]. At the same time, GO sheets are partially reduced to rGO at 170 oC. In the second step, the strongly coupled hybrid ZnCo2O4 QDs/rGO can be obtained by simply thermal annealing treatment in 6

air at 250 oC. The morphology and microstructure of the obtained ZnCo-glycolate/rGO and ZnCo2O4 QDs/rGO2 were examined by FESEM and TEM under different magnifications. As shown in Figure 2a-b, two-dimensional nanostructures of ZnCo-glycolate/rGO can be observed after refluxing at 170 oC for 2 h. Magnified TEM image demonstrates a rough surface, which derives from ZnCo-glycolate anchored on the surface of crumpled rGO sheet (Supporting Information, Figure S1a). Similar to that of ZnCo-glycolate/rGO, sheet-like morphology can be obtained after thermal annealing treatment (Figure 2c-d). No unattached nanocrystals are observed, indicating strong interactions between ZnCo2O4 QDs and rGO. A representative TEM image in Figure 2e displays the ultrafine ZnCo2O4 nanocrystals are homogeneously and densely distributed on the surface of rGO sheets. The uniform distribution of ZnCo2O4 QDs is further confirmed by energy-dispersive spectroscopy (EDS) elemental mapping. Figure 3 depicts the typical SEM images of the hybrids (Figure 3a and 3b) and elemental mapping images of C (Figure 3c), Zn (Figure 3d), Co (Figure 3e) and O (Figure 3f) from the same area, in which the uniform distribution of Zn, Co, O and C is experimentally mapped out. The particle sizes of ZnCo2O4 nanocrystals are about 2-5 nm (Supporting Information, Figure S1b). In the absence of GO, only irregular nanoparticles could be obtained (Supporting Information, Figure S1c). It is postulated that homogeneous distribution of ultrafine ZnCo2O4 QDs would not only be beneficial for a good utilization of the electrical conductivity of rGO, but also provide large electrochemically active sites for Li+ shuttling in/out of hybrids, leading 7

to high-performance lithium storage capability [15,54]. The high-resolution TEM (HRTEM) image of ZnCo2O4 QDs is presented in Figure 2f, in which the interplanar distance of lattice fringes is measured to be 0.24 nm, corresponding to the (311) crystal plane of spinel ZnCo2O4 [45,55]. Inset in Figure 2f shows that the selected-area electron diffraction (SAED) pattern of ZnCo2O4 possesses well-defined diffraction rings, suggesting the polycrystalline nature of the resultant nanocrystals [16]. The XRD pattern of the ZnCo-glycolate/rGO hybrids depicts that only weak peaks can be observed (Supporting Information, Figure S2a). A broad peak at around 10 o can be ascribed to the partially reduced graphene oxide due to the weak reducing ability of ethylene glycol at 170 oC [53], while other peaks with quite low intensity are derived from the ZnCo-glycolate [28]. Figure 4a shows the XRD patterns of the ZnCo2O4 QDs/rGO with different mass ratios of ZnCo2O4. All the diffraction peaks can be indexed with the standard XRD patterns of cubic ZnCo2O4 spinel phase (JCPDS card no. 23-1390) without noticeable signals of impurity peaks [44]. No characteristic diffraction peak of GO is observed in the XRD patterns of ZnCo2O4 QDs/rGO, indicating the successful reduction of GO [56]. With the increase of ZnCo2O4 content in the ZnCo2O4 QDs/rGO hybrids, the peak intensity of the crystalline ZnCo2O4 becomes stronger (Supporting Information, Figure S2b). As shown in the Raman spectrum of ZnCo2O4 QDs/rGO2 (Supporting Information, Figure S3), the peaks centered at 477, 528, and 675 cm-1 are attributed to vibrational modes of spinel phases [15,57], while the peaks at 1350 and 1601 cm-1 are ascribed to 8

the D and G bands of rGO, respectively [15]. Commonly, the intensity ratio (ID/IG) of the D band to G can serve as a measurement of defects in carbon materials. A larger ID/IG of ZnCo2O4 QDs/rGO2 compared with that of rGO indicates that there are more defects for Li storage [57]. From the TGA analyses, the mass ratios of ZnCo2O4 in the ZnCo2O4 QDs/rGO1, ZnCo2O4 QDs/rGO2 and ZnCo2O4 QDs/rGO3 are about 73.6%, 85.0% and 91.1% respectively. In order to examine the chemical composition and oxidation state of sample, XPS is performed and corresponding XPS data are shown in Figure 4c-f. The survey spectrum shows the existence of Zn, Co, O and C elements in ZnCo2O4 QDs/rGO2. Figure 4d depicts the high-resolution Zn 2p spectrum. There exist two peaks centered at binding energies (BEs) of 1044.3 and 1021.2 eV, attributed to Zn 2p1/2 and Zn 2p3/2, respectively, which indicate the Zn(II) in the ZnCo2O4 phase [19]. In the Figure 4e, the peaks located at BEs of 794.8 and 779.9 eV can be ascribed to the Co 2p1/2 and Co 2p3/2, respectively. The BE separation between these two peaks is about 14.9 eV, which is in line with Co(III) oxidation state of ZnCo2O4 [16,20]. The high-resolution spectrum of O 1s is deconvoluted into two peaks, as shown in Figure 4f. One peak at 531.1 eV is attributed to C-O and C=O derived from rGO, another peak at 529.6 eV corresponds to the metal-oxygen bonds [39]. Therefore, the XPS results confirm the formation of ZnCo2O4/rGO2 hybrid and support the XRD data. The Brunauer-Emmett-Teller (BET) specific surface area (SSA) and porous feature of samples are investigated by nitrogen adsorption/desorption isotherms. Figure 5a shows the isotherm of ZnCo2O4 QDs/rGO2, which can be classified as type 9

IV isotherm with type H4 according to the IUPAC classification. According to the investigation results, the BET SSA of ZnCo2O4 QDs/rGO2 is 125.8 m2 g-1, almost tripling the SSA of pure ZnCo2O4 (44.4 m2 g-1). Figure 5b depicts the Barrett-Joyner-Halenda (BJH) pore-size distribution (PSD) datum of ZnCo2O4 QDs/rGO2. Obviously, the derived PSD demonstrates that the majority of the pores are in the range of 1.8-30 nm. The average pore size of ZnCo2O4 QDs/rGO2 is 9.5 nm. The existence of mesopores could greatly fast the diffusion of Li+, and supply sufficient void space for effectively buffering the volume variation during discharge/charge processes [28,48]. The electrochemical properties of the ZnCo2O4 QDs/rGO1, ZnCo2O4 QDs/rGO2, ZnCo2O4 QDs/rGO3 and ZnCo2O4 are evaluated by cyclic voltammetry (CV) and galvanostatic discharge/charge measurements for lithium storage. Figure 6a shows the first three CV curves of ZnCo2O4 QDs/rGO2 at a scan rate of 0.1 mV s-1 in the voltage range of 0.01-3.0 V. Consistent with the CV curves of pure ZnCo2O4 anode (Supporting Information, Figure S4), several redox current peaks of ZnCo2O4 QDs/rGO2 could be identified from the CV curves, indicating the similar lithium storage mechanism [28,31]. The curve of the first cycle in the cathodic process is substantially different from the second one, indicating the existence of some irreversible reduction reactions during the first discharge. In the first cycle, the cathodic peak located at 0.6 V can be assigned to the reduction reaction of ZnCo2O4 with Li into metallic Zn and Co. In the anodic scan, two broad oxidation peaks centered at about 1.69 V and 2.12 V correspond to the oxidation of Zn to Zn2+ and Co 10

to Co3+, respectively [44]. During the second cycle, the cathodic peak shifts to higher potentials, which might be attributed to the activation process for the Li+ insertion in the first cycle [45]. After that, the subsequent CV curves almost overlap each other, implying the excellent reversibility of the electrochemical reactions. Based on the above analyses and previous reported lithium storage mechanisms of ZnO and Co3O4, the electrochemical reactions for our ZnCo2O4 QDs/rGO2 electrode can be proceed as follows [25,30]: ZnCo2O4 + 8 Li+ + 8 e → Zn + 2 Co + 4 Li2O (1) Zn + Li+ + e ↔ LiZn (2) Zn + Li2O ↔ ZnO + 2 Li+ + 2 e (3) 2 Co + 2 Li2O ↔ 2 CoO + 4 Li+ + 4 e (4) 2 CoO + 2/3 Li2O ↔ 2/3 Co3O4 + 4/3 Li+ + 4/3 e (5) Typical galvanostatic discharge/charge profiles of ZnCo2O4 QDs/rGO2 at a current density of 200 mA g-1 between 0.01 and 3.0 V are revealed in Figure 6b. From the voltage profiles, the first discharge curve of ZnCo2O4 QDs/rGO2 electrode shows a well-defined long voltage plateau between 0.8-1.0 V, followed by a gradual voltage decrease to the cutoff potential of 0.01V, which is attributed to the reduction of ZnCo2O4 to Zn and Co, followed by the alloying of Zn with Li to LiZn [44]. The initial discharge and charge capacities of ZnCo2O4 QDs/rGO2 are 1575.7 and 1052.7 mAh g-1 respectively, much higher than that of ZnCo2O4 QDs/rGO1, ZnCo2O4 QDs/rGO3 and ZnCo2O4 (Supporting Information, Figure S5). On the basis of discharge/charge capacities, the Coulombic efficiency of ZnCo2O4 QDs/rGO2 during 11

the first cycle is 66.8%. The corresponding irreversible loss of about 33.2% is due to the following reasons. First, the formation of solid electrolyte interphase (SEI) layer traps lithium ions. Second, the conversion reaction of ZnCo2O4 is partially reversible [39,45]. However, the discharge/charge curves approximately overlap except for the initial discharge profile, indicating excellent reversibility of the electrode material for lithium storage. To evaluate the cycling stability, the hybrids and pure ZnCo2O4 are discharged and charged at a current density of 500 mA g-1 for 100 cycles in the voltage range of 0.01 to 3.0 V, as shown in Figure 6c. The hybrids display much higher reversible capacities and better cycling performances than pure ZnCo2O4. The discharge capacity of pure ZnCo2O4 is only 483.5 mAh g-1 after 100 cycles, corresponding to 59.1% of the second-cycle discharge capacity. The poor cycling capability of pure ZnCo2O4 is attributed to the large volume expansion and aggregation during the Li+ insertion/extraction processes, resulting in the loss of electrical contact between electrode material and current collector [16,24]. The ZnCo2O4 QDs/rGO2 has shown escalation in discharge capacity upon cycles after the initial capacity dropping. This capacity rise has also been observed in other TMOs and is likely attributed to the activation process as well as the reversible growth of polymeric gel-like film resulting from electrolyte degradation [15,31,32]. The specific capacity of ZnCo2O4 QDs/rGO2 is 1062 mAh g-1 after 100 cycles, which is higher than that of ZnCo2O4 QDs/rGO1 (822.5 mAh g-1), ZnCo2O4 QDs/rGO3 (601.5 mAh g-1), and other previously reported ZnCo2O4 materials, such as ZnCo2O4 tube-in-tube (560 mAh g-1 at 200 mA g-1 after 12

70 cycles) [21], yolk-shelled ZnCo2O4 microspheres (718 mAh g-1 at 500 mA g-1 after 100 cycles) [28], leaf-like ZnCo2O4/Ni foam electrode (850 mAh g-1 at 416 mA g-1 after 50 cycles) [29], ZnCo2O4 microspheres/NiSix nanowires (890 mAh g-1 at 500 mA g-1 after 100 cycles) [33], nano-sized ZnCo2O4/graphene (755.6 mAh g-1 at 0.1 C after 70 cycles) [44], and ZnCo2O4 nanosheets/rGO (960.8 mAh g-1 at 90 mA g-1 after 100 cycles) [45]. The large capacity and excellent cycling performance of ZnCo2O4 QDs/rGO2 is attributed to large specific surface area and 2D mesoporous nanostructure [39]. Large surface area of electrode shortens Li+ diffusion distance and increase the utilization of electrode, leading to high capacity. Moreover, 2D mesoporous structure could hold the structural integrity through strong interaction between ZnCo2O4 QDs and flexible rGO demonstrated by preservation of the integrity of structure after 100 discharge-charge cycles at 500 mA g-1 (Figure 7), which might result in excellent cycling stability [15,39]. Besides high specific capacity and good cycle stability, the rate capability is also an important parameter for high-performance LIBs. The rate properties of ZnCo2O4 QDs/rGO hybrids and pure ZnCo2O4 are cycled for each ten cycles under the current densities from 200 to 3600 mA g-1, as shown in Figure 6d. As expected, ZnCo2O4 QDs/rGO2 hybrid exhibits enhanced capacity retention in comparison with pure ZnCo2O4 as the current density increases. The capacities for ZnCo2O4 QDs/rGO2 electrode are 1027, 944.8, 778.1, 676.1 and 491.2 mAh g-1 at the densities of 200, 400, 800, 1600, 3200 mA g-1. It should be note that even at a high density of 3200 mA g-1, the capacity is still much higher than the theoretical capacity of graphite (372 mAh 13

g-1). As the current density turns back to 200 mA g-1, the capacity of the ZnCo2O4 QDs/rGO2 hybrid is still as high as 1025.2 mAh g-1 after the rate performance test, much better than that of pure ZnCo2O4 (577.1 mAh g-1). Notably, the excellent capability of ZnCo2O4 QDs/rGO2 is also superior to that of other previous works, such as ZnCo2O4 microspheres (435 mAh g-1 at 2000 mA g-1) [19], flower-like ZnCo2O4 nanowires (347 mAh g-1 at 800 mA g-1) [20], ZnO/ZnCo2O4 rod array (700 mAh g-1 at 445 mA g-1) [22], yolk-shelled ZnCo2O4 microspheres (647 mAh g-1 at 1500 mA g-1) [28], and ZnCo2O4 nanosheets/rGO (593.2 mAh g-1 at 900 mA g-1) [45]. The long-term cyclability measurement of as-prepared ZnCo2O4 QDs/rGO2 is tested as an anode for LIBs at higher current of 2000 mA g-1. The capacity is 682.5 mAh g-1 after 1000 cycles, along with the Coulombic efficiency close to 100% (Figure 8a). The capacity gradually decreases to 401.1 mAh g-1 in the first 81 cycles, and then increases to high value in the following 190 cycles. This phenomenon is often observed in some transition metal oxides, and generally attributed to electrochemical activation of electrode as well as heterogeneous storage of Li at the developing nanodomain and/or nanophase interfaces with extensive interfacial areas [15,31,39]. To understand the reason for the enhanced electrochemical performance of the ZnCo2O4 QDs/rGO2 hybrid, electrochemical impedance spectra of ZnCo2O4 QDs/rGO2 and pure ZnCo2O4 before cycling are performed in the frequency range from 10 mHz to 100 KHz, as shown in Figure 8b. Apparently, the diameter of semicircle for ZnCo2O4 QDs/rGO2 in the high- to medium-frequency region is much 14

smaller than that of pure ZnCo2O4, which demonstrates that the contact and charge-transfer resistances are lower for ZnCo2O4 QDs/rGO2 hybrid [39]. At the low frequency region, the slope of ZnCo2O4 QDs/rGO2 is larger than that of pure ZnCo2O4, suggesting the faster Li+ ion diffusion behavior of hybrid electrode [31]. It is the faster electron transfer and ion diffusion that result in outstanding improvement on the electrochemical performance of our ZnCo2O4 QDs/rGO2 electrode. The outstanding electrochemical performance of ZnCo2O4 QDs/rGO2 can be attributed to synergic effect between rGO sheets and ZnCo2O4 QDs (inset in Figure 8b). On one hand, the ultrafine ZnCo2O4 QDs can not only supply extra electrochemically active sites for Li+ insertion, and decrease the restacking of the neighboring graphene sheets at the same time, leading to the high reversible capacity, but also effectively short diffusion path length for Li+, resulting in excellent rate capability [15,31]. On the other hand, the rGO sheets can act as superior conductively network to increase the electrical conductivity of the hybrid, fasting transport of charge between ZnCo2O4 QDs and current collector [39]. In addition, the highly elastic and stable rGO sheets can accommodate volume expansion/contraction of ZnCo2O4 QDs during discharging/charging, which is beneficial for cycle life [58-59]. More importantly, the strongly coupled interaction can further restrain the volume change and aggregation of ZnCo2O4 QDs, which consequently contribute to the enhanced electrochemical performance [15,39]. 4. Conclusions In the present work, strongly coupled ZnCo2O4 QDs/rGO2 with mesoporous 15

structure has been prepared via a simple polyol method and followed by thermal annealing treatment in air. The ZnCo2O4 QDs/rGO2 demonstrates a high reversible capacity of 1062 mAh g-1 over 100 cycles at the current density of 500 mA g-1. The specific capacities are 1027, 944.8, 778.1, 676.1 and 491.2 mAh g-1 at the densities of 200, 400, 800, 1600, 3200 mA g-1, respectively. Furthermore, as cycling at the higher current density of 2000 mA g-1 up to 1000 cycles, the capacity can still maintain a value as high as 682.5 mAh g-1. Thus it is believed that this hybrid could be a promising anode material for high-performance LIBs. ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jiangsu Province (BK20140473 and BK20131220), Scientific Research Foundation for Returned Scholars, Ministry of Education of China and Research fund of Yancheng Institute of Technology (KJC2013006). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version. References [1] J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan, X. Lou, Recent advances in metal oxide-based electrode architecture design for electrochemical energy etorage, Adv. Mater. 24 (2012) 5166. [2] H. Wang, H. Dai, Strongly coupled inorganic-nano-carbon hybrid materials for energy etorage, Chem. Soc. Rev. 42 (2013) 3088. [3] K. Wang, X. Li, J. Chen, Surface and interface engineering of electrode materials 16

for lithium-ion batteries, Adv. Mater. 27 (2015) 527. [4] S. Yu, S.H. Lee, D.J. Lee, Y. Sung, T. Hyeon, Conversion reaction-based oxide nanomaterials

for

lithium

ion

battery

anodes,

Small

2015

DOI:

10.1002/smll.201502299. [5] K. Rui, Z. Wen, Y. Lu, J. Jin, C. Shen, One-step solvothermal synthesis of nanostructured manganese fluoride as an anode for rechargeable lithium-ion batteries and insights into the conversion mechanism, Adv. Energy Mater. 5 (2015) 1401716. [6] H. Li, L. Shen, B. Ding, G. Pang, H. Dou, X. Zhang, Ultralong SrLi2Ti6O14 nanowires composed of single-crystalline nanoparticles: promising candidates for high-power lithium ions batteries, Nano Energy 13 (2015) 18. [7] X. Huang, R. Wang, D. Xu, Z. Wang, H. Wang, J. Xu, Z. Wu, Q. Liu, Y. Zhang, X. Zhang, Homogeneous CoO on graphene for binder-free and ultralong-life lithium ion batteries, Adv. Funct. Mater. 23 (2013) 4345. [8] X.Wang, Y. Fan, R.A. Susantyoko, Q. Xiao, L. Sun, D. He, Q. Zhang, High areal capacity Li ion battery anode based on thick mesoporous Co3O4 nanosheet networks, Nano Energy 5 (2014) 91. [9] K. Zhang, W. Zhao, L. Lee, G. Jang, X. Shi, J.H. Park, A magnetic field assisted self-assembly strategy towards strongly coupled Fe3O4 nanocrystal/rGO paper for high-performance lithium ion batteries, J. Mater. Chem. A 2 (2014) 9636. [10] H. Zhang, L. Zhou, O. Noonan, D.J. Martin, A.K. Whittaker, C. Yu, Tailoring the void size of iron oxide@carbon yolk-shell structure for optimized lithium storage, Adv. Funct. Mater. 24 (2014) 4337. 17

[11] X. Gu, J. Yue, L. Chen, S. Liu, H. Xu, J. Yang, Y. Qian, X. Zhao, Coaxial MnO/N-doped carbon nanorods for advanced lithium-ion battery anodes, J. Mater. Chem. A 3 (2015) 1037. [12] H. Wang, L. Cui, Y. Yang, H.S. Casalongue, J.T. Robinson, Y. Liang, Y. Cui, H. Dai, Mn3O4-graphene hybrid as a high-capacity anode material for lithium ion batteries. J. Am. Chem. Soc. 132 (2010) 13978. [13] X. Liu, J. Cheng, W. Li, X. Zhong, Z. Yang, L. Gu, Y. Yu, Superior lithium storage in a 3D macroporous graphene framework/SnO2 nanocomposite, Nanoscale 6 (2014) 7817. [14] Z. Li, J. Ding, H. Wang, K. Cui, T. Stephenson, D. Karpuzov, D. Mitlin, High rate SnO2-graphene dual aerogel anodes and their kinetics of lithiation and sodiation, Nano Energy 15 (2015) 369. [15] P. Xiong, B. Liu, V. Teran, Y. Zhao, L. Peng, X. Wang, G. Yu, Chemically integrated two-dimensional hybrid zinc manganate/graphene nanosheets with enhanced lithium storage capability, ACS Nano 8 (2014) 8610. [16] A.K. Giri, P. Pal, R. Ananthakumar, M. Jayachandran, S. Mahanty, A.B. Panda, 3D hierarchically assembled porous wrinkled-paper-like structure of ZnCo2O4 and Co-ZnO@C as anode materials for lithium-ion batteries. Cryst. Growth Des. 14 (2014) 3352. [17] B. Liu, X. Wang, B. Liu, Q. Wang, D. Tan, W. Song, X. Hou, D. Chen, G. Shen, Advanced

rechargeable

lithium-ion

batteries

based

on

ZnCo2O4-urchins-on-carbon-fibers electrodes, Nano Res. 6 (2013) 525. 18

bendable

[18] Z. Sun, W. Ai, L. Liu, X. Qi, Y. Wang, J. Zhu, H. Zhang, T. Yu, Facile fabrication of hierarchical ZnCo2O4/NiO core/shell nanowire arrays with improved lithiumion battery performance, Nanoscale, 6 (2014) 6563. [19] L. Hu, B. Qu, C. Li, Y. Chen, L. Mei, D. Lei, L. Chen, Q. Li, T. Wang, Facile synthesis of uniform mesoporous ZnCo2O4 microspheres as a high-performance anode material for Li-ion batteries, J. Mater. Chem. A 1 (2013) 5596. [20] S.G. Mohamed, T. Hung, C. Chen, C.K. Chen, S. Hu, R. Liu, K. Wang, X. Xing, H. Liu, A. Liu, M. Hsieh, B. Lee, Flower-like ZnCo2O4 nanowires: toward a high performance anode material for Li-ion batteries. RSC Adv. 3 (2013) 20143. [21] G. Zhang, B.Y. Xia, C. Xiao, L. Yu, X. Wang, Y. Xie, X.W. Lou, General formation of complex tubular nanostructures of metal oxides for the oxygen reduction reaction and lithium-ion batteries, Angew. Chem. Int. Ed. 52 (2013) 8643. [22] C.W. Lee, S. Seo, D.W. Kim, S. Park, K. Jin, D. Kim, K.S. Hong, Heteroepitaxial growth of ZnO nanosheet bands on ZnCo2O4 submicron rods toward high-performance Li ion battery electrodes, Nano Res. 6 (2013) 348. [23] B. Liu, J. Zhang, X. Wang, G. Chen, D. Chen, C. Zhou, G. Shen, Hierarchical three-dimensional ZnCo2O4 nanowire arrays/carbon cloth anodes for a novel class of high-performance flexible lithium-ion batteries, Nano Lett. 12 (2012) 3005. [24] H. Yu, C. Guan, X. Rui, B. Ouyang, B. Yadian, Y. Huang, H. Zhang, H.E. Hoster, H.J. Fan, Q. Yan, Hierarchically porous three-dimensional electrodes of CoMoO4 and ZnCo2O4 and their high anode performance for lithium ion batteries, Nanoscale 6 (2014) 10556. 19

[25] B. Qu, L. Hu, Q. Li, Y. Wang, L. Chen, T. Wang, High-performance lithium-ion battery anode by direct growth of hierarchical ZnCo2O4 nanostructures on current collectors. ACS Appl. Mater. Interfaces 6 (2014) 731. [26] Y. Sharma, N. Sharma, G.V.S. Rao, B.V.R. Chowdari, Nanophase ZnCo2O4 as a high performance anode material for Li-ion batteries, Adv. Funct. Mater. 17 (2007) 2855. [27] N. Du, Y. Xu, H. Zhang, J. Yu, C. Zhai, D. Yang, Porous ZnCo2O4 nanowires synthesis via sacrificial templates: high-performance anode materials of Li-ion batteries, Inorg. Chem. 50 (2011) 3320. [28] J. Li, J. Wang, D. Wexler, D. Shi, J. Liang, H. Liu, S. Xiong, Y. Qian, Simple synthesis

of

yolk-shelled

ZnCo2O4

microspheres

towards

enhancing

the

electrochemical performance of lithium-ion batteries in conjunction with a sodium carboxymethyl cellulose binder, J. Mater. Chem. A 1 (2013) 15292. [29] H. Long, T. Shi, S. Jiang, S. Xi, R. Chen, S. Liu, G. Liao, Z. Tang, Synthesis of a nanowire self-assembled hierarchical ZnCo2O4 shell/Ni current collector core as binder free anodes for high-performance Li-ion batteries, J. Mater. Chem. A 2 (2014) 3741. [30] Q. Xie, F. Li, H. Guo, L. Wang, Y. chen, G. Yue, D. Peng, Template-free synthesis of amorphous double-shelled zinc-cobalt citrate hollow microspheres and their transformation to crystalline ZnCo2O4 microspheres, ACS Appl. Mater. Interfaces 5 (2013) 5508. [31] J. Bai, X. Li, G. Liu, Y. Qian, S. Xiong, Unusual formation of ZnCo2O4 3D 20

hierarchical twin microspheres as a high-rate and ultralong-life lithium-ion battery anode material, Adv. Funct. Mater. 24 (2014) 3012. [32] S.H. Choi, Y.C. Kang, Yolk-shell, hollow, and single-crystalline ZnCo2O4 powders: preparation using a simple one-pot process and application in lithium-ion batteries, ChemSusChem 6 (2013) 2111. [33] H. Chen, Q. Zhang, J. Wang, Q. Wang, X. Zhou, X. Li, Y. Yang, K. Zhang, Mesoporous ZnCo2O4 microspheres composed of ultrathin nanosheets cross-linked with metallic NiSix nanowires on Ni foam as anodes for lithium ion batteries, Nano Energy 10 (2014) 245. [34] C. Yuan, H.B. Wu, Y. Xie, X.W. Lou, Mixed transition-metal oxides: design, synthesis, and energy-related applications, Angew. Chem. Int. Ed. 53 (2014), 1488. [35] H. Guo, L. Liu, T. Li, W. Chen, J. Liu, Y. Guo, Y. Guo,

Accurate hierarchical

control of hollow crossed NiCo2O4 nanocubes for superior lithium storage, Nanoscale 6 (2014) 5491. [36] J. Li, S. Xiong, Y. Liu, Z. Ju, Y. Qian, High electrochemical performance of monodisperse NiCo2O4 mesoporous microspheres as an anode material for Li-ion batteries, ACS Appl. Mater. Interfaces 5 (2013) 981. [37] G. Gao, H.B. Wu, X.W. Lou, Citrate-assisted growth of NiCo2O4 nanosheets on reduced graphene oxide for highly reversible lithium storage, Adv. Energy Mater. 4 (2014) 1400422. [38] L. Shen, L. Yu, X. Yu, X. Zhang, X.W. Lou, Self-templated formation of uniform NiCo2O4 hollow spheres with complex interior structures for lithium-ion 21

batteries and supercapacitors, Angew. Chem. Int. Ed. 54 (2015) 1868. [39] W. Yao, J. Xu, J. Wang, J. Luo, Q. Shi, Q. Zhang, Chemically integrated multiwalled carbon nanotubes/zinc manganate nanocrystals as ultralong-life anode materials for lithium-ion batteries, ACS Sustainable Chem. Eng. 3 (2015) 2170. [40] G. Zhang, L. Yu, H.B. Wu, H.E. Hoster, X.W. Lou, Formation of ZnMn2O4 ball-in-ball hollow microspheres as a high-performance anode for lithium-ion batteries, Adv. Mater. 24 (2012) 4609. [41] Y. Dong, Y, Xia, Y. Chui, C. Cao, J.A. Zapien, Self-assembled three-dimensional mesoporous ZnFe2O4-graphene composites for lithium ion batteries with significantly enhanced rate capability and cycling stability, J. Power Sources 275 (2015) 769. [42] G. Zhang, X.W. Lou, General synthesis of multi-shelled mixed metal oxide hollow spheres with superior lithium storage properties, Angew. Chem. Int. Ed. 53 (2014) 9041. [43] G. Zhang, H.B. Wu, H.E. Hoster, X.W. Lou, Strongly coupled carbon nanofiber-metal oxide coaxial nanocables with enhanced lithium storage properties, Energy Environ. Sci. 7 (2014) 302. [44] A.K. Rai, T.V. Thi, B.J. Paul, J. Kim, Synthesis of nano-sized ZnCo2O4 anchored with graphene nanosheets as an anode material for secondary lithium ion batteries, Electrochim. Acta 146 (2014) 577. [45] G. Gao, H.B. Wu, B. Dong, S. Ding, X.W. Lou, Growth of ultrathin ZnCo2O4 nanosheets on reduced graphene oxide with enhanced lithium storage properties, Adv. 22

Sci. 2 (2015) 1400014. [46] J. Wang, D. Jin, R. Zhou, C. Shen, K. Xie, B. Wei, One-step synthesis of NiCo2S4 ultrathin nanosheets on conductive substrates as advanced electrodes for high-efficient energy storage, J. Power Sources 306 (2016) 100. [47] X. Song, Q. Ru, Y. Mo, L. Guo, S. Hu, B. An, A novel porous coral-like Zn0.5Ni0.5Co2O4 as an anode material for lithium ion batteries with excellent rate performance, J. Power Sources 269 (2014) 795. [48] C. Yuan, L. Zhang, L. Hou, L. Zhou, G. Pang, L. Lian, Scalable room-temperature synthesis of mesoporous nanocrystalline ZnMn2O4 with enhanced lithium storage properties for lithium-ion batteries, Chem. Eur. J. 21 (2015) 1262. [49] S. Liu, Y. Dong, C. Zhao, Z. Zhao, C. Yu, Z. Wang, J. Qiu, Nitrogen-rich carbon coupled multifunctional metal oxide/graphene nanohybrids for long-life lithium storage and efficient oxygen reduction, Nano Energy 12 (2015) 578. [50] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (2011) 780. [51] H. Xia, C. Hong, B. Li, B. Zhao, Z. Lin, M. Zheng, S.V. Savilov, S.M. Aldoshin, Facile synthesis of hematite quantum-dot/functionalized graphene-sheet composites as advanced anode materials for asymmetric supercapacitors, Adv. Funct. Mater. 25 (2015) 627. [52]

H.

Zhou,

W.

Yao,

G.

Li,

J.

Wang,

Y.

Lu,

Graphene/

poly(3,4-ethylenedioxythiophene) hydrogel with excellent mechanical performance 23

and high conductivity, Carbon 59 (2013) 495. [53] G. Zhang, B.Y. Xia, X. Wang, X.W. Lou, Strongly coupled NiCo2O4-rGO hybrid nanosheets as a methanol-tolerant electrocatalyst for the oxygen reduction reaction, Adv. Mater. 26 (2014) 2408. [54] Y. Chen, B. Song, L. Lu, J. Xue, Ultra-small Fe3O4 nanoparticle decorated graphene nanosheets with superior cyclic performance and rate capability, Nanoscale 5 (2013) 6797. [55] H. Chen, Q. Zhang, J. Wang, Q. Wang, X. Zhou, X. Li, Y. Yang, K. Zhang, Mesoporous ZnCo2O4 microspheres composed of ultrathin nanosheets cross-linked with metallic NiSix nanowires on Ni foam as anodes for lithium ion batteries, Nano Energy 11 (2015) 687. [56] W. Yao, T. Ni, S. chen, H. Li, Y. Lu, Graphene/Fe3O4@polypyrrole nanocomposites as a synergistic adsorbent for Cr(VI) ion removal, Compos. Sci. Technol. 99 (2014) 15. [57] D. Wang, W. Zhou, Y. Zhang, Y. Wang, G. Wu, K. Yu, G. Wen, A novel one-step strategy toward ZnMn2O4/N-doped graphene nanosheets with robust chemical interaction for superior lithium storage, Nanotechnology 27 (2016) 045405. [58] D. Cai, D. Wang, H. Huang, X. Duan, B. Liu, L. Wang, Y. Liu, Q. Li, T. Wang, Rational synthesis of ZnMn2O4 porous spheres and graphene nanocomposite with enhanced performance for lithium-ion batteries, J. Mater. Chem. A 3 (2015) 11430. [59] K. Zhao, L. Zhang, R. Xia, Y. Dong, W. Xu, C. Niu, L. He, M. Yan, L. Qu, L. Mai, SnO2 quantum dots@graphene oxide as a high-rate and long-life anode material 24

for lithium-ion batteries, Small 12 (2016) 588.

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Figure 1. Schematic diagram of the synthetic route to ZnCo2O4 QDs/rGO.

Figure 2. (a) FESEM and (b) TEM images of ZnCo-glycolate/rGO. (c, d and e) TEM images of the ZnCo2O4 QDs/rGO. (f) High-resolution TEM image of the ZnCo2O4 QDs/rGO (inset: the corresponding selected-area electron diffraction pattern of ZnCo2O4 QDs).

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Figure 3. (a and b) SEM images of ZnCo2O4 QDs/rGO hybrid and the corresponding EDS mapping images of (c) carbon, (d) zinc, (e) cobalt and (f) oxygen.

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Figure 4. (a) XRD patterns and (b) TGA curves of ZnCo2O4 QDs/rGO1, ZnCo2O4 QDs/rGO2 and ZnCo2O4 QDs/rGO3. XPS spectra of ZnCo2O4 QDs/rGO2: (c) survey spectrum, (d) Zn 2p, (e) Co 2p and (f) O1s.

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Figure 5. (a) Nitrogen adsorption/desorption isotherms and (b) pore size distributions of ZnCo2O4 QDs/rGO2 and ZnCo2O4.

Figure 6. (a) Cyclic voltammograms of ZnCo2O4 QDs/rGO2 at a scan rate of 0.1 mV s-1. (b) Galvanostatic discharge/charge curves of ZnCo2O4 QDs/rGO2 at a current density of 200 mA g-1. (c) Cycling performance of ZnCo2O4 QDs/rGO1, ZnCo2O4 QDs/rGO2, ZnCo2O4 QDs/rGO3 and ZnCo2O4 at a current density of 500 mA g-1. (d) Rate capability of ZnCo2O4 QDs/rGO1, ZnCo2O4 QDs/rGO2, ZnCo2O4 QDs/rGO3 and ZnCo2O4 at different densities of 200, 400, 800, 1600, and 3200 mA g-1. 29

Figure 7. (a) and (b) FESEM images of ZnCo2O4 QDs/rGO2 after 100 discharge-charge cycles at 500 mA g-1.

Figure 8. (a) Cycling performance and Coulombic efficiency of ZnCo2O4 QDs/rGO2 electrode at a current density of 2000 mA g-1 for more than 1000 cycles. (b) Electrochemical impedance spectra (EIS) of ZnCo2O4 QDs/rGO2 and pure ZnCo2O4 (inset: schematic of the lithium storage advantage of ZnCo2O4 QDs/RGO2).

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