Elsevier

Electrochimica Acta

Volume 197, 10 April 2016, Pages 58-67
Electrochimica Acta

Highly ordered mesoporous spinel ZnCo2O4 as a high-performance anode material for lithium-ion batteries

https://doi.org/10.1016/j.electacta.2016.03.047Get rights and content

Abstract

Highly-ordered mesoporous spinel ZnCo2O4 (HOM-ZnCo2O4) with controlled morphology, high surface area and narrow pore size distribution is prepared through a facile infiltration route with molecular sieve SBA-15 as templates for the first time. At the same time, nanostructured porous spherical spinel ZnCo2O4 (NPS-ZnCo2O4) framework is also synthesized using silicon spheres as templates. HOM-ZnCo2O4 is revealed to have a highly ordered mesoporous structure with a pore diameter of ∼4 nm and a high surface area of 112.0 m2 g−1, while NPS-ZnCo2O4 is a framework containing 3D connected pores with a diameter of ∼100 nm and surface area of 88.4 m2 g−1. As the anode material, HOM-ZnCo2O4 displays a high reversible specific capacity up to 1623 mA h g−1, ∼300 mA h g−1 larger than NPS-ZnCo2O4 (1286 mA h g−1), at a current density of 2.0 Ag−1. When applying a high current density of 8.0 A g−1, the capacity of HOM-ZnCo2O4 still remains at a high level of 1470 mA h g−1, but NPS-ZnCo2O4 undergoes a severe degradation to 751 mA h g−1. The large specific surface area contributes much to the better rate performance of the former because it provides a larger cross section for Li+ flux. Also, the small pore size may be more in favor of maintaining the structural stability of porous spinel material than the large one. Aside from its nanostructured characteristics, an inner atomic synergistic effect within the cubic lattices may account for the superior electrochemical performance of HOM-ZnCo2O4. HOM-ZnCo2O4 that exhibits high capacities and well stability is a kind of competitive anode materials and highly-ordered mesoporous structure is superior in electrode design.

Introduction

Nowadays, the miniaturization of portable electronics puts forward higher requirements for the power source, lithium-ion batteries (LIBs), in terms of energy density and powder density [1], [2], [3], [4], [5]. As we know, current commercial LIBs usually employ graphite as the anode material, which is becoming the short board in pursing higher battery performance due to its relatively low theoretical capacity (372 mA h g−1). To break such limit, intensive research efforts have been devoted to develop alternative anode materials to replace graphite [6], [7]. Metal oxides exhibit high theoretical capacity and appear as promising candidates to meet this requirement, mainly based on two mechanisms, i.e., alloying/dealloying or conversion reaction. For the former, it mainly includes Sn, In, Si based oxides [8], [9], [10], and for the latter, it involves oxides of Fe, Co, Ni, Cu, etc. [11], [12], [13], [14]. Following the reaction mechanism, the high lithiation degree, e.g., Li/Sn can reach 4.4 for SnO2, Li/Co is up to 8/3 for Co3O4, much higher than 1/6 of graphite (produce LiC6), should be responsible for the high capacity of metal oxides. Co-based oxides, such as CoO, Co2O3, Co3O4, etc.13 have been widely investigated as LIB anode materials since Poizot et al.’s report in 2000 [14]. Among them, Co3O4 is the most promising one to replace graphite because of its high theoretical capacity (892 mA h g−1) and relatively easy preparation process. However, the practical application of Co3O4 was still shrouded by several problems, for example, (i) the pulverization of anode materials from current collectors due to the huge volume change in charge and discharge process, (ii) low Li+ diffusion kinetics, and (iii) the toxicity and high cost of Co element [15], [16].

Up to now, partially replacing Co with cost-effective and eco-friendly elements, such as Zn, Fe, Ni, Cu, etc. appeared as a promising strategy to mitigate the predicament. Compared with Fe, Ni, Cu that have variable valence, Zn was easier to introduce a controllable change in Co3O4 due to its invariable bivalent state. In fact, Zn2+ only replaced bivalent Co2+ occupying the tetrahedral sites (8a) and Co3+ ions located at octahedral sites (16d), forming a compound of ZnCo2O4 with “normal” spinel structure [17], [18], [19], [20], [21]. ZnCo2O4 can store Li+ not only through conversion reaction of Co oxide, but also including the alloying/dealloying reaction between Zn and Li, as well as the nanostructured characteristics and the possibly inner atomic synergistic effect of Zn- and Co-based components [22], [23], which results in a higher theoretical capacity of ∼976 mA h g−1 than pristine Co3O4 (892 mA h g−1) [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. As a result, ZnCo2O4 has received much attention as the anode material. Nevertheless, the former two problems were still not well solved only by such replacement. Methods that were adopted before to enhance the cycling stability and Li+ diffusion speed have been widely employed to design ZnCo2O4 anode structure, aiming to reach a package improvement of above problems. Typically, various nanostructures of ZnCo2O4 such as porous nanowires [24], [27], [32], [33], nanoparticles [25], [30], porous nanoflakes [26], microspheres [28], [31], porous nanotubes [29], and nanorods [30] have been prepared as anode materials and exhibited well performance. For example, Bai et al. reported that two ZnCo2O4 micro/nanostructures displayed better capacity retention, exceptionally high rate performance and ultralong cyclic stability [28]. Porous ZnCo2O4 nanotubes prepared by an electrospinning method followed by a thermal annealing procedure in air, exhibited high capacity, good cyclability, and superior rate capability [29]. Qu et al. prepared a 3D ZnCo2O4 hierarchical nanostructure on a Ni foam to form a conductivity-agent-free and binder-free electrode suitable for use as the anode of LIBs [33]. Actually, the stability and Li+ diffusion kinetics strongly depended on the microstructure and morphology of the electrodes, where nano dimensions and porous structure were quite favorable since they provided short Li+ diffusion path and much space to hold the volume expansion. Here, to our surprise was that there were few reports on ordered mesoporous ZnCo2O4 as an anode, not like the case in other oxides, e.g., TiO2, MnO2, etc [11], [12], [13], [14], [16]. The mesoporous structure possesses ordered hole channels [35], which was able to accommodate the electrolyte and facilitate the Li+ diffusion to the inner active sites of anode materials [36], and a large specific surface area, providing a large electrolyte/electrode contact region for getting a high Li+ flux across the interface [37]. Also, nanosized pores could serve to buffer the volume expansion and alleviate the strain [38]. Such unique advantages make mesoporous structure probably the first choice for the designing of anode [39], [40]. However, to the best of our knowledge, there are almost no reports on the synthesis of the highly crystalline ordered mesoporous spinel ZnCo2O4 nanorods and further used as anodes for lithium ion batteries in addition to some mesoporous spheres with disordered pore structure.

In this work, spinel HOM-ZnCo2O4 materials with a narrow pore size distribution, a high pore volume and a high specific surface area were first prepared by a facile infiltration route using SBA-15 as templates [9]. The electrochemical performance demonstrated that the developed HOM-ZnCo2O4 electrode displays considerably outstanding higher coulombic efficiency, better capacity retention and rate capability compared with the ordinary porous spinel ZnCo2O4 materials, e.g., NPS-ZnCo2O4. The HOM-ZnCo2O4 materials displayed a high specific capacity up to 1674.8 mA h g−1 after 200 cycles, even at a current density of 0.5 A g−1, with good cycling stability. We also found that small pores were favorable to keep structural stability than large ones, and large surface area played a vital role in getting the high rate capability. The synthesized highly ordered mesoporous spinel ZnCo2O4 materials here served as a promising anode material for lithium ion batteries.

Section snippets

Synthesis of template

SBA-15 template was synthesized according to the reference [41]. 3 g of triblock copolymer P123, 90 mL of dilute hydrochloric acid (2 M) and 24 mL of deionized water were transferred into a flat bottom flask with vigorous stirring for 30 min. Then 6.25 g of TEOS was added to the above solution and kept at 40 °C for 24 h, followed by heated to 100 °C for 24 h. The product was filtered and washed three times with DI water and dried in an oven at 80–100 °C for 8 h. Finally, after calcination in air at 550 °C

Structural analysis of the spinel ZnCo2O4 Samples

Small- and wide-angle XRD examination were carried out to characterize the porous structure and phase of the synthesized products as shown in Fig. 1. From Fig. 1a, the small-angle XRD pattern exhibits two weak peaks, corresponding to (110) and (200) planes, respectively, which are characteristic of the two-dimensional hexagonal mesostructure with P6 mm space group. It indicates that texture of HOM-ZnCo2O4 materials is highly ordered, actually is homogenous nanorod arrays with uniform holes

Conclusion

We prepared highly ordered mesoporous spinel ZnCo2O4 material with SBA-15 as templates for the first time and used them as anode materials. It indicated that spinel ZnCo2O4 was a promising anode material because of its very large capacity (maximum of 1943 mA h g−1 in this work). If carefully designing the electrodes into proper porous structure, their structure stability, rate capability and capacity could be evidently improved. Comparison between HOM-ZnCo2O4 and NPS-ZnCo2O4 further demonstrated

Acknowledgements

We acknowledge support from the project supported by the Sate Key Program of National Natural Science of China (No.: 51532005), National Nature Science Foundation of China (No.: 51472148, 51272137), the Tai Shan Scholar Foundation of Shandong Province, and the Fundamental Research Funds of Shandong University.

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