Synthesis of multimodal porous ZnCo2O4 and its

Journal of Power Sources 294 (2015) 112e119

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Synthesis of multimodal porous ZnCo2O4 and its electrochemical properties as an anode material for lithium ion batteries Shiji Hao a, b, Bowei Zhang b, Sarah Ball c, Mark Copley c, Zhichuan Xu b, *, Madhavi Srinivasan b, *, Kun Zhou d, Subodh Mhaisalkar b, Yizhong Huang b, * a

Energy Research Institute @ NTU, Nanyang Technological University, Singapore 639798, Singapore School of Materials Science & Engineering, Nanyang Technological University, Singapore 639798, Singapore Johnson Matthey Technology Centre, Reading RG4 9NH, UK d School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore b c

h i g h l i g h t s ZnCo2O4 multimodal porous microspheres are prepared via solvothermal method. Two-step formation mechanism of flower like precursor is proposed. ZnCo2O4 delivers high rate capability and long lifespan.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 April 2015 Received in revised form 19 May 2015 Accepted 9 June 2015 Available online 19 June 2015

In the present paper, flower-like multimodal porous ZnCo2O4 microspheres, comprised of numerous nanosheets, are synthesized through PVP assist solvothermal self-assembling process. The multimodal porous ZnCo2O4 microspheres are characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). A possible formation mechanism of two steps self-assemble is proposed. The ZnCo2O4 microspheres are then used as an anode material to fabricate lithium ion batteries. The results based on the evaluation of lithium ion batteries demonstrate that the porous microstructure offers the excellent electrochemical performance with high capacity and long-life cycling stability. It is found that a high reversible capacity of 940 and 919 mAh g1 is maintained after 100 cycles at a low chargeedischarge rate of 0.1C and 0.2C (100 and 200 mA g1), respectively. Meanwhile, the remaining discharging capacity reaches as high as 856 mAh g1 after 1000 cycles subject to the large current density up to 1C. © 2015 Elsevier B.V. All rights reserved.

Keywords: ZnCo2O4 Multimodal porosity Anodes Lithium ion batteries

1. Introduction Recently, the urgent demand of lithium ion batteries (LIBs) applied in electric vehicles (EVs) and hybrid electric vehicles (HEVs) powering system requires the development of novel anode materials with high performance [1e3]. Transition metal oxides (such as ZnO [4,5], Mn3O4 [1,6], CuO [7,8], Fe3O4 [9,10] and Co3O4 [11,12]) have attracted much attention as the most promising next generation LIB anode materials. Among all of these transition metal oxides, Co3O4 shows relatively high capacity (890 mAhg1, two times higher than graphite (372 mAhg1)) [12], and is a potential

* Corresponding authors. E-mail address: [email protected] (Y. Huang). http://dx.doi.org/10.1016/j.jpowsour.2015.06.048 0378-7753/© 2015 Elsevier B.V. All rights reserved.

candidate for LIBs. However, it has a key disadvantage of being toxic. The efficient way to solve the problem is to partially replace cobalt with some other transition metals that are eco-friendly and cheap, forming transition metal oxide such as CoFe2O4 [13], CuCo2O4 [14] and ZnCo2O4 [15,16]. ZnCo2O4 is of particular interest for LIBs since it has a theoretical capacity of approximately more than 900 mAhg1 resulted by the alloying-de-alloying reaction between Zn and Li [17]. So far, many ZnCo2O4 based materials with different structures (hollow [18], yolk-shell [19], porous [20], nanowire [16] and et al.) have been developed in order to further enhance the LIB capacity performance. However, similar to other transition metal oxides, the critical issue of ZnCo2O4 materials for LIBs is the drastic volume change

S. Hao et al. / Journal of Power Sources 294 (2015) 112e119

during lithiation/delithiation processes resulting in the severe capacity fading after several discharge/charge cycles [21]. The effective way to overcome this challenge is to build up and design a wide variety of nanostructures such as metal oxide/carbon matrix composite [22], thin films [23], hollow structures [24] and mesoporous structures [25]. Among all these nanostructures, 3 dimensional (3D) mesoporous structures are very attractive due to their shorter lithium ion path by facilitating electrolyte contact with the pores along with the extra free space for accommodating volume change during cycling [26]. More recently, many 3D ZnCo2O4 based materials with mesoporosities have been synthesized and reported as high performance anode electrode for LIBs [26e28]. Nevertheless, Yu et al.’s research showed that the carbon based multimodal porous anode materials provide a better cycling performance and rate capability than the mesoporous anodes [29]. The multimodal porosities with macropores enable the stability enhancement of the morphology under the effect of buffer reservoir and allow a fast mass and electron transport. Considerable effort has been focused on the development of transition oxide based anode materials with multimodal porosity nanostructures. As an example, Li and co-workers prepared the multimodal porous MnO/carbon displaying high performance for LIBs' anode electrode [30]. Xie et al. also reported that the hematite spheres with hierarchical pores showed great activity in LIBs [31]. However, to the best of my knowledge, there have been very few reports on the preparation of ZnCo2O4 based anode materials with 3D multimodal porosity for LIBs. In the present work, we propose a facile approach to synthesize multimodal porous ZnCo2O4 microspheres under assisting of polyvinylpyrrolidone (PVP). The evaluation of ZnCo2O4 microspheres as an anode material for LIBs, demonstrates their performance much better than other ZnCo2O4 based anode materials as reported in literature previously [27,32e34]. It is believed that the excellent performance is attributed to the unique structure which is

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multimodal porous and able to offer high specific surface area, large surface-to-volume ratio and improved structural stability against volume expansion. As a result, ion/electron transfer rate and sufficient contact interface between active materials and electrolyte are enhanced. 2. Experimental 2.1. Materials synthesis All the purchased chemicals (SigmaeAldrich) are used in analytical grades without purification. A solvothermal method was used to synthesize the multimodal porous ZnCo2O4 microsphere precursors. The synthesis starts with dissolving 1 g PVP (5,5000 MW) into 30 ml ethanol using ultrasonication for at least 20 min. Subsequently, 1 mmol Zn(NO3)2$4H2O, 2 mmol Co(NO3)2$4H2O and 7 mmol Urea were added into the above solution, followed by ultrasonication and stirring until they were thoroughly dissolved. After that, the suspension was sealed in a Teflon-lined stainless-steel autoclave (50 ml). The autoclave was heated to 160 C and remained for 3 h before being cooled to room temperature. The green precipitate was collected by centrifugation (less than 3000 rpm) and then washed with ethanol and DI water for several times and dried at 60 C in an oven for overnight. Finally, the porous ZnCo2O4 microspheres were obtained after the precursor was annealed at 600 C for 5 h in air (2 C min1). 2.2. Characterization The microstructure of the final ZnCo2O4 product and the pristine precursor were examined by using powder X-ray diffraction (XRD, Bruker D8 with Cu Ka radiation), scanning electron microscopy (SEM, JEOL 6340) and Transmission electron microscopy (TEM, JEOL 2100). The thermogravimetric analyses (TGA) were conducted on

Fig. 1. SEM image of the as-obtained ZnCo2O4 precursor.

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overnight to remove the water content. The pure lithium foils and Celgard 2400 membrane were used as counter electrodes and the separator, respectively. The galvanostatic chargeedischarge cycling test was conducted at 25 C between 0.01 V and 3 V under different rates (0.1C, 0.2C, 0.5C, 1C, 2C and 5C) using a Neware battery test system (Neware, BTS-5V10 mA, China) and the cell was aged for more than 8 h before testing. Cyclic voltammetry (CV) test was performed by using an electrochemical workstation (autolab, PGSTAT302N) with a scan rate of 0.1 mV s1 over 0.1 Ve3.0 V (vs. Li/ Liþ) applied to the test cells. 3. Results and discussion

Fig. 2. TGA (blue solid) and the corresponding derivative curve (black solid) curves for the precursor in flowing air. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the TA Instruments DMA Q800. The BrunauereEmmetteTeller (BET) specific surface areas of the ZnCo2O4 particles were measured by using the Micromeritics ASAP Tristar II 3020. The X-ray photoelectron spectroscopy (XPS, Omicron analyzer EA 125) was used to analyze the surface element electron state. The milling and crosssection view of precursor microspheres is conducted by Focus Ion Beam system (FIB, FEI Nova Nanolab 600i).

2.3. Electrochemical measurement ZnCo2O4 electrodes for the electrochemical measurement were prepared by coating the slurry of the 70% active materials, 20% super p-carbon and 10% polyvinylidene fluoride (PVDF) on copper foil which was then left overnight to dry up at 60 C. 1 M LiPF6 of ethylene carbonate (EC) and Diethyl carbonate (DEC) (1:1 by volume) solution was used as the electrolyte. The assembly of the test cells was operated in an argon-filled glove box (moisture＜1 ppm) before the working electrodes were dried under vacuum at 60 C

Well-dispersed microspheres with an average diameter of 1e1.2 mm are clearly seen in Fig. 1, a SEM image showing the surface morphology of ZnCo2O4 precursor. The ZnCo2O4 Precursor microsphere is made up by numerous flakes which give the product rough surface and porous interior as shown in the high magnification SEM images (Fig. 1b,c). The interior structure is viewed from the cross-section of a ZnCo2O4 microsphere (Fig. S1) milled and imaged in a FIB system. TGA analysis was performed to characterize the thermal properties of the ZnCo2O4 precursor and the decomposition process during the heat treatment. As shown in Fig. 2, there are two weight losses: the first weight loss below 200 C is due to the loss of physical and chemical adsorption of ethanol, water or other organic species, while the second weight loss of 23% between 210 and 240 C is attributed to the decomposition of the precursor to ZnCo2O4, as indicated by a sharp peak (black solid derivative curve). To ensure the complete decomposition of the precursor, calcination temperature of 600 C was chosen. Fig. 3c shows the XRD pattern of the final product after annealing at 600 C for 5 h. All the reflections can be indexed to face-centered-cubic [35] ZnCo2O4 (JCPDS no. 231390) with a spinel structure. No additional diffraction peaks from impurity are detectable, suggesting the high purity of the final product. The microstructures of the ZnCo2O4 particles after calcination

Fig. 3. (a and b) SEM image and (c) XRD pattern of the as-synthesized ZnCo2O4 and XRD after calcination under 600 C for 5 h.


under 600 C for 5 h were examined using SEM (Fig. 3a, b) and TEM (Fig. 4). As shown in Fig. 3a, after heating, the flakes constructing ZnCo2O4 particles are seen thicker and shorter but particles still remain the microspherical shape resulting in a denser structure. The thickness of flakes was measured from the high magnification SEM image (Fig. 3b) to be around 100 nm in average, three time more than the original flakes (around 30 nm) before annealing. Fig. 4a, a dark-field TEM image of ZnCo2O4 after annealing, presents a 1.2 mm sized porous microsphere, which comprises interconnected ZnCo2O4 nanocrystals with an average diameter of around 50 nm (Fig. 4b, a zoomed image taken from Fig. 4a in the square - marked area). The perfection of nanocrystals are demonstrated by the high-resolution TEM image (Fig. 4c imaged at a zone axis of [2 2 0], determined from Fig. 4d, where the adjacent interplanar distance of 0.285 nm measured corresponses to the spacing between {220} planes of ZnCo2O4 crystals). The elemental distribution on the ZnCo2O4 microsphere (Fig. 4e) was performed using energy-dispersive X-ray spectroscopy (EDS) in scanning TEM mode. The presence of the elements Zn, Co and O are shown in Fig. 4feh, respectively, which distribute uniformly across the microspeherical

Fig. 4. (a) Dark-field TEM and (b,c,d) HRTEM images of the as-synthesized ZnCo2O4 particle and (e) elemental mapping.

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particle. In addition, X-ray-photoelectron-spectroscopy (XPS) analysis was also carried out in order to investigate the surface electronic states and the composition of the as-synthesized ZnCo2O4 particles with the testing results shown in Fig. 5. The C 1s line with the binding energy of 284.60 eV is used as the reference for calibration. The survey spectrum (Fig. 5a) shows the presence of Zn, Co, and O as well as C. The two major peaks [36] (at the binding energies of 1044.4 eV and 1021.3 eV) in Fig. 5b are attributed to Zn 2p1/2 and Zn 2p3/2 of Zn2þ. The binding energy values of the two major peaks in the Co 2p spectrum are 780.3 and 795.3 eV respectively, are associated with Co 2p3/2 and Co 2p1/2 peaks [18]. It is noted that the spineorbit splitting of the two peaks is 15 eV, in accordance with the data reported in the literature [37,38]. Two accompanied weak satellite peaks are visible at 790.0 eV and 805.0 eV and the energy gap between the main peak and the satellite peaks is around 9.7 eV. This suggests that the Co cation valence can be assigned a value of þ3 [39]. In the O 1s spectrum (Fig. 5d), the refined peaks at 529.9 and 531.7 eV are attributed to the lattice oxygen from the ZnCo2O4 particles and the oxygen from hydroxide ions, respectively. The additional peak at around 533.0 eV is believed to be generated from a small amount of physically adsorbed water molecules [40,41]. The specific surface areas and porous nature of the ZnCo2O4 microspheres were investigated with BET gas-sorption measurements. The isotherm curve is shown in Fig. 6, which can be categorized as type IV isotherm with H3 hysteresis loop corresponding to the existence of mesopores. The results show that ZnCo2O4 particles possess a high bet specific area of 77.23 m2 g1 and a pore volume of 0.419 cm3 g1. And using BarretteJoynereHalenda BJH calcination (inset of Fig. 6), the pore size distribution plots is demonstrated. The figure exhibits the large specific area mainly deriving from the existence of mesopores (2e50 nm) and macropores (>50 nm) corresponding to a Broaden Pore size distribution ranging from 5 to 40 nm and a small narrow peak centered at 70 nm, respectively. The multimodal porosity is owed to the unique structure of ZnCo2O4 microspheres. The porous structure not only increases the interfacial contact between the electrolyte and the active metal but also offers the buffer area for the ZnCo2O4 particle volume change during lithiation and delithiation cycling. In order to understand the formation process of the ZnCo2O4 precursor, the time-dependent experiments were carried out, in which samples were collected at different time intervals (i.e. 10 min, 30 min, 1 h and 2 h). Many nanosheets with the diameter of about 1 mm were produced (Fig. 7a) when the solvothermal reaction was conducted for 10 min. These nanosheets began to assemble into around 1 mm flower like microspheres (Fig. 7b) after 20 min' time. With longer time reaction, nanosheets grew thicker and became nanoplatelets (Fig. 7c). The further growth of nanoplatelets gave rise to the formation of compact microspheres (Fig. 7d). No uniformly-assembled microspheres were synthesized in experiment without PVP as shown in Fig. S2. It was found that PVP plays a key role in the production of the flower like structure. This is due to the carbonyl groups of PVP that weaken the interaction with cations in the solution and also are selectively adsorbed on the specific crystal plane of the primary particles, as reported previously [42,43]. A certain amount of PVP assists the assembly of the nanoplates into flower structure microspheres through molecular interaction between surfactant molecules. The formation of flower like ZnCo2O4 porous precursor microsphere can be proposed as a two-step growth model [44e46]. The growth mechanism is similar to the formation process of hematite with flower like nanostructure, as reported previously [47]. It can be schematically illustrated in Fig. 7e. Firstly, ethanol coordinates with Zn(NO3)2 and Co(NO3)2 producing Zinc Cobalt alkoxide precipitate which then acts as the nuclei leading to the quick growth of

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Fig. 5. XPS spectra: (a) survey spectrum, (b) Zn 2p, (c) Co 2p, and (d) O 1s for the as-synthesized ZnCo2O4 particles.

primary particles with high surface energy [48]. Subsequently, the presence of Urea allows these primary particles to be aggregated spontaneously into nanosheets through the process of Ostwald ripening driven by the minimization of interfacial energy, as proved by SEM image of the 10 min precursor product. With the assistance of PVP, These nanosheets are self-assembled into flower like hierarchical structure based on their inner crystallographic orientation. Finally, the continuous growth of hierarchical branches of microspheres under the effect of Ostwald ripening mechanism results in the formation of the compact assembled microspheres. The as-synthesized ZnCo2O4 was assembled into laboratorybased CR2016 coin cells to investigate the electrochemical performance by cyclic voltammetry (CV) and galvanostatic dischargecharge measurements. The 1st, 30th and 60th discharge

Fig. 6. BET isotherm plots and corresponding BJH pore size distributions (insets) of ZnCo2O4.

(Li þ insertion) and charge (Li þ extraction) profiles were performed at a rate of 0.1C (100 mA g1) in the voltage window of 0.01e3 V (Fig. 8a). In the first cycle, the broad and steady discharging platform occurred at around 1 V after which the voltage gradually decreases. The initial discharge and charge capacities were measured to be 1438 and 1102 mAh g1, respectively. The irreversible capacity loss of the first cycle (23.37%) is likely attributed to the formation of solid electrolyte interphase (SEI) and the reduction of metal oxide to metal with Li2O formation. Meanwhile, the discharge capacity of ZnCo2O4 in the 30th and 60th are calculated to be approximately 1256 mAh g1 and 1129 mAh g1, respectively. The discharging potential plateau located at around 1 V disappears after 1st cycling but appears a long discharging potential slope between 1.4 V and 0.75 V. It seems that the potential slop possesses a fairly steady location in the following cycles from the good superimposition of the charge and discharge curves after the first cycle. It strongly indicates that a stable SEI film forms on the active metals during first cycle, and the inherent porous structure of the as-obtained ZnCo2O4 particles offers a good electrochemical reversibility and stability of the working electrode in the lithium insertion and extraction reactions under relatively low rate condition (0.1C). Based on the previous studies [17,35], the electrochemical reactions involved in the discharge and charge processes are believed to proceed as follows: ZnCo2O4 þ 8Liþ þ 8e / Zn þ 2Co þ 4Li2O

(1)

Zn þ Liþ þ e 4 LiZn

(2)

Zn þ Li2O 4 ZnO þ 2Liþ þ 2e

(3)

2Co þ 2Li2O 4 2CoO þ 4Liþ þ 4e

(4)


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Fig. 7. SEM images of the ZnCo2O4 precursors collected at different intervals (a) 10 min (b) 30 min (c) 1 h and (d) 2 h. (e) Schematic illustration of the morphological evolution process of the ZnCo2O4 precursor.

2CoO þ 2/3Li2O 42/3Co3O4 þ 4/3 Liþ þ 4/3e

(5)

CV measurement was carried out at a scan rate of 0.1 mV s1 in the voltage window of 0.01e3.0 V. As shown in Fig. 8b, a sharp reduction peak located at around 0.86 V indwelled in the first discharge cycle, which corresponds to the reduction of ZnCo2O4 to Zn and Co (Equation (1)). In the anodic part of the first scan, two broad oxidation peaks are visible at 1.67 V and 2.1 V, which are assigned to the oxidation of Co0 and Zn0 to Co3þ and Zn2þ, respectively (Equations (3)e(5)) [20]. In the following 2nd, 3rd and 4th cycles, the cathodic peak is gradually shifted to 0.9 V and becomes much broader, indicating that different electrochemical mechanisms govern the two processes [19]. Similar shifts in anodic/ cathodic peaks voltage is observed in the CV curve of ZnCo2O4 with different nanostructure [49e51]. The overlap of the CV curves after 1st cycle implies the excellent stability and cyclability of ZnCo2O4 electrode. Fig. 8c shows the electrochemical performance of the ZnCo2O4 electrode examined by galvanostatic discharge-charge tests at 0.1C between 0.01 and 3 V. It is found that the as-synthesized ZnCo2O4 provides excellent cycling property. The discharge and charge capacity gradually goes up from the initial cycle until the 37th cycle and reaches the maximum where the respective discharge and

charge capacity are 1285 and 1265 mAh g1, respectively. It is quite obvious that the value of reversible capacity is much higher than the theoretical number (975 mAh g1). The extra capacity is contributed to two parts. First, the reversible formation/dissolution of polymeric/gel-like films on the surface of active materials resulted from electrolyte degradation brings about the extra capacity, which has been observed by Laruelle et al. [52]. And Second, metal nanoparticles could be formed in the discharging process, and then these fresh metal surface area which generated from the conversion reaction of ZnCo2O4 during lithiation generally could cause additional capacity by electrocatalysing C4þ in the Li2CO3 (the one of major ingredient in SEI film) to other C forms with lower valences along with the formation of Li2O and the chemical energy transfer for energy storage, as reported by Su et al. [53]. What's more, the capacity increasing in the first several cycling is mainly attributed to the presence of mesoporous structure, according to the study of Shaju et al. [54]. In the subsequent cycles, the capacity is still maintained around 1000 mAh g1. After 100 cycles, the discharge capacity is as high as 926 mAh g1. In the whole cycle range, the coulombic efficiency is above 97% except the 1st cycle (76.6%), indicating the excellent cycling stability of the working electrode contributed from the active materials' unique porous structure. The porous structure of the as-synthesized ZnCo2O4

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particles could offers a fast diffusion path for the electrolyte, increase the specific active surface area and act as buffer area for the volume change. The high rate capability of the ZnCo2O4 electrode was evaluated by multiple-step galvanostatic chargeedischarge test. The results are shown in Fig. 8d where the rate capabilities are measured to be 1150, 1040, 900, 780, 670 and 500 mAh g1, respectively, at five rates (0.1C, 0.2C, 0.5C, 1C, 2C and 5C). It is noted that even the lowest capacity of high rate (5C) still shows much higher value than the theoretical capacity of graphite (372 mAh g1). More importantly, a discharge capacity of 809 mAh g1 is achieved and ramps up to more than 1000 mAh g1 within 1 cycle after the high rate discharge-charge cycles. The cycling behavior of the ZnCo2O4 electrode was further investigated at both low and high rates of 0.2C and 1C, respectively. As shown in Fig. 8e, f, the initial discharge capacities of active metal at 0.2C and 1C both show relatively high value of 1370 and 1209 mAh g1, respectively. In the situation of 0.2C, the slight growth of capacities occurs in the first 25 cycles and then the capacities gradually decrease to the final discharging capacity of 919 mAh g1, after 100 cycles. The discharging capacity at the rate

of 1C is as high as 856 mAh g1 after 1000 cycles. It's worth to note that the capacity decreases down to 690 mAh g1 after 130 cycles. It subsequently increases to around 850 mAh g1 at a slow rate until the 380th cycle and remains stable in the following cycles. This phenomenon has been widely observed in transition metal oxide based anode materials. The increase of the capacity is attributed to the formation of the polymeric/gel-like films by kinetically activated electrolyte degradation [52,53]. In the present work, a unique multimodal porous hierarchical architecture is introduced, which demonstrates a great electrochemical performance for anode materials of LIBs. On the one hand, the multimodal porous structure offers large active surface area for electrochemical reactions and enables to promote the integrated chargeedischarge state leading to the effective increase of the specific capacity of the active material. The unique multimodal porosities, on the other hand, allows the penetration of electrolyte and offers fast diffusion pathway for Li ion and mass transfer contributing to the increase of capacity. More importantly, the macro porosities provide buffer volume for the volume change during the conversion reaction of Lithium intercalation and extraction causing the enhancement of cycling performance.

Fig. 8. (a) Discharge and charge curves of the as-prepared ZnCo2O4 microspheres; (b) the first four cyclic voltammogram (CV) curves of the ZnCo2O4 microspheres at a scan rate of 0.1 mV s1; (c) cycling performance of the electrode made of the ZnCo2O4 microspheres at a current of 0.1C; (d) cycling performance of the ZnCo2O4 microspheres at various current densities; (e) and (f) cycling performance of the ZnCo2O4 microspheres at a current of 0.2C and 1C, respectively.


4. Conclusion In summary, we have synthesized multimodal ZnCo2O4 microspheres through PVP assisted solvothermal synthesis combining post calcination treatment. The precursor appears a flowe-like microsphere structure assembled from multiple nanosheets and is conversed to a multimodal porous ZnCo2O4 microspheres after heat treatment. The multimodal porous ZnCo2O4 microspheres were tested as an anode material in LIBs, which demonstrates reasonably high specific capacity and good cyclability under high rate condition. The excellent electrochemical performance of the as-obtained ZnCo2O4 is attributed to the unique porous microstructure. It can not only accommodate the volume change/ expansion during working as anode but also offer fast Li ion diffusion pathway and large active surface area to increase the specific capacity. Acknowledgments This research was support by SUG (Start-up funding in NTU), Tier 1 (AcRF grant MOE Singapore M401992), Tier 2 (AcRF grant MOE Singapore M4020159), NTU-JM joint grant (M4061392) and the Chinese Natural Science Foundation (Grant 51271031). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.06.048. References [1] H.L. Wang, L.F. Cui, Y.A. Yang, H.S. Casalongue, J.T. Robinson, Y.Y. Liang, Y. Cui, H.J. Dai, J. Am. Chem. Soc. 132 (2010) 13978e13980. [2] G. Derrien, J. Hassoun, S. Panero, B. Scrosati, Adv. Mater. 19 (2007) 2336. [3] C.K. Chan, H.L. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol. 3 (2008) 31e35. [4] X.H. Huang, X.H. Xia, Y.F. Yuan, F. Zhou, Electrochim Acta 56 (2011) 4960e4965. [5] K.T. Park, F. Xia, S.W. Kim, S.B. Kim, T. Song, U. Paik, W.I. Park, J. Phys. Chem. C 117 (2013) 1037e1043. [6] J. Gao, M.A. Lowe, H.D. Abruna, Chem. Mater. 23 (2011) 3223e3227. [7] S. Ko, J.I. Lee, H.S. Yang, S. Park, U. Jeong, Adv. Mater. 24 (2012) 4451e4456. [8] B. Wang, X.L. Wu, C.Y. Shu, Y.G. Guo, C.R. Wang, J. Mater. Chem. 20 (2010) 10661e10664. [9] E. Kang, Y.S. Jung, A.S. Cavanagh, G.H. Kim, S.M. George, A.C. Dillon, J.K. Kim, J. Lee, Adv. Funct. Mater. 21 (2011) 2430e2438. [10] G.M. Zhou, D.W. Wang, F. Li, L.L. Zhang, N. Li, Z.S. Wu, L. Wen, G.Q. Lu, H.M. Cheng, Chem. Mater. 22 (2010) 5306e5313. [11] N. Yan, L. Hu, Y. Li, Y. Wang, H. Zhong, X.Y. Hu, X.K. Kong, Q.W. Chen, J. Phys. Chem. C 116 (2012) 7227e7235. [12] Z.S. Wu, W.C. Ren, L. Wen, L.B. Gao, J.P. Zhao, Z.P. Chen, G.M. Zhou, F. Li, H.M. Cheng, ACS Nano 4 (2010) 3187e3194. [13] Z.H. Li, T.P. Zhao, X.Y. Zhan, D.S. Gao, Q.Z. Xiao, G.T. Lei, Electrochim Acta 55 (2010) 4594e4598. [14] Y. Sharma, N. Sharma, G.V.S. Rao, B.V.R. Chowdari, J. Power Sources 173 (2007) 495e501. [15] Y.C. Qiu, S.H. Yang, H. Deng, L.M. Jin, W.S. Li, J. Mater. Chem. 20 (2010) 4439e4444. [16] B. Liu, J. Zhang, X.F. Wang, G. Chen, D. Chen, C.W. Zhou, G.Z. Shen, Nano Lett.

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