Strongly coupled hybrid ZnCo2O4 quantum dots/reduced graphene oxide with high-performance lithium storage capability
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], [2], [3], [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], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], NiCo2O4 [35], [36], [37], [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 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 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.
Section snippets
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 °C 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 were annealed at 250 °C for 2 h in air
Results and Discussion
The proposed procedure for the preparation of the ZnCo2O4 QDs/rGO is illustrated in Fig. 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 °C. In the second step, the strongly coupled hybrid ZnCo2
Conclusions
In the present work, strongly coupled ZnCo2O4 QDs/rGO2 with mesoporous 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
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).
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