Synthesis of hierarchical ZnO/ZnCo₂O₄ nanosheets with mesostructures for lithium-ion anodes

Abstract

Novel bi-component-active hierarchical ZnO/ZnCo₂O₄ (ZZCO) nanosheets with mesostructures are successfully prepared through a convenient and practical hydrothermal route followed by a calcination process. When evaluated as anode materials for lithium-ion batteries (LIBs), the bi-component-active hierarchical ZZCO nanosheets with mesostructures exhibit highly reversible lithium storage capacity, and strong cycling stability at the 500 mA g⁻¹ rate for 150 cycles. More significantly, the hybrid ZZCO presents an exceptionally high rate capability up to the 2 A g⁻¹ rate.

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Transition metal oxides with high theoretical capacity have received much attention to render them promising candidates of anode materials for lithium ion batteries (LIBs).¹ There are two types of oxides systems in transition metal oxides. One type of oxides (such as Co, Cu, Fe, Ni, and Mn based oxides) as lithium-ion anodes can form Li₂O and nanocrystalline metal nanoparticles through a redox reaction during the Li⁺ insertion/extraction process. The other types of oxides (Sn, In and Zn based oxides) as lithium-ion anodes can react with Li to form an alloyed metal phase during the Li⁺ insertion/extraction process,¹ providing extra discharge capacity. AB₂O₄ spinel (or spinel-like) structured oxides contain the above mentioned two types of transition metals (A and B), A and B have an oxidation state of +2 and +3, respectively. The AB₂O₄ spinel (or spinel-like) structured oxides have generated increasing interest as promising lithium-storage materials for LIBs.¹ Among AB₂O₄ spinel (or spinel-like) structured oxides, ZnCo₂O₄ has attracted much attention because of its low toxicity, low cost, high reversible capacities, strong cycling stability and high thermal stability.² However, the poor electrical conductivity, electrode pulverization and large volume change of ZnCo₂O₄ hinder its development space during the cycle process as anode materials for LIBs.

In recent years, great efforts have been made to overcome this problem. One approach is to develop novel nanostructure electrode material (such as hierarchical architecture, hollow structure or mesostructures). The mesoporous structure is widely used in LIBs because of its large capacities, high surface area, and low density. Importantly, mesoporous structure can accommodate the volume expansion during the Li⁺ insertion/extraction process; heighten surface-to-volume ratio and reduce Li ion diffusion length.³ As an important class of nanostructures, there-dimensional (3D) hierarchical architectures own large surface areas, numerous conducting channels for electron transport and more active sites,² which have become a research hotspot as electrode materials. Hierarchical architectures can facilitate the Li ion diffusion due to a reduced diffusion length within the active material particles and an increased electrode/electrolyte contact area.⁴ It is noteworthy that hierarchical mesostructure possesses the advantages of hierarchical architecture and mesostructures, which can availably relieve agglomeration, buffer the volume expansion, provide satisfactory electrochemical properties.^5,6 Therefore, it is highly desirable to develop novel hierarchical mesostructure as high performance anodes for LIBs.

The other avenue is to hybridize nanocomposites.⁷ Nanohybrids are able to reveal strong synergistic effects of electrochemical performance, which allow us to make the best of inherent interaction of each component.⁸ More importantly, nanohybrids are capable of presenting larger specific capacity and more stable cycling performance than any single component.⁹ The hybridization of bi-component metal oxides, such as Co₃O₄/Fe₂O₃,¹⁰ NiFe₂O₄/Fe₂O₃,⁷ and ZnO/ZnFe₂O₄,¹¹ which all demonstrate that these unique nanohybrid anodes exhibiting a strong synergistic effect of high capacity and remarkable rate capability than any single one.^7,8 However, CoO has a low theoretical capacity of ∼750 mA h g⁻¹ and suffers from poor capacity retention during cycling and/or poor rate capability. It is important that cobalt is toxic, expensive and high lithium redox potential (2.2–2.4 V vs. Li⁺/Li). Thus, CoO/ZnCo₂O₄ is not suitable for lithium-ion anode materials. By contrast, ZnO is eco-friendly, cheaper and the alloying–dealloying reaction between Zn and Li, resulting in a high theoretical capacity of 900 mA h g⁻¹. Consequently, synthesis of bi-component hierarchical metal oxides (contain ZnO) with mesostructures is crucial to advance the development of LIBs. However, it is extremely challenging to establish an integrated hierarchical architecture using a simple and facile approach.

In this study, we report the rational design and synthesis of novel bi-component-active hierarchical ZnO/ZnCo₂O₄ (ZZCO) nanosheets with mesostructures via a convenient and practical hydrothermal route followed by a calcination process. Importantly, the hybrid ZZCO delivers high reversible lithium storage capacity and excellent rate capability, which should be attributed to novel hierarchical mesostructures and synergetic effect of the ZnO and ZnCo₂O₄ can facilitate Li⁺ ion transport by reducing paths both for Li⁺ ions and electrons diffusion. Moreover, this synthetic strategy can be expanded as a general approach to prepare binary oxides with hierarchical mesostructures.

The crystallographic structure and phase purity of the as-prepared samples are tested by powder X-ray diffraction (XRD) measurement. The XRD pattern of the as-prepared ZnO/ZnCo₂O₄ (ZZCO) materials is shown in the Fig. 1a. All the diffraction peaks exhibit the two-phase coexistence of the cubic ZCO (ZnCo₂O₄) with a spinel structure (JCPDS card no. 23–1390) and hexagonal ZnO (JCPDS card no. 36–1451) phases,¹² implying the formation of the hybrid ZZCO. No any other patterns are observed, indicating the high purity of the as-prepared samples. Energy dispersive spectroscopy (EDS) microanalysis of the hybrid ZZCO materials (Fig. 1b) shows that the whole micro/nanostructures of as-prepared samples include only Zn, Co, and O three elements, further confirming the formation of hybrid ZZCO.

Fig. 1 XRD (a) and EDS (b) patterns of the as-prepared ZZCO materials.

The detailed morphology and microstructure of hybrid ZZCO materials are further characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The typical SEM images of the as-prepared ZZCO samples are shown in Fig. S1† and 2. The Fig. S1† reveals that a typical sample composed of abundant dispersive ZZCO samples flower-like microspheres with irregular diameters from 5 μm to 10 μm. The Fig. 2a and b clearly reveal that these highly hierarchical ZZCO microspheres are composed of numerous porous nanosheets. Detailed structural features of ZZCO microspheres are further characterized by TEM. The TEM images (Fig. 2a and b) show that the ZZCO flower-like microspheres are assembled from thin porous nanosheets, to show highly hierarchical structure. From the Fig. 2e can clearly see numerous porous on the nanosheets of ZZCO. ZCO samples are also assembled from porous nanosheets through changing solvent and temperature in the synthesis process. The XRD pattern and SEM images of as-prepared ZCO samples are shown in Fig. S2 and S3.†

Fig. 2 SEM (a and b) and TEM (c–e) images of the as-prepared ZZCO materials.

The purity, surface electronic states and chemical composition of the ZZCO samples are further confirmed by X-ray photoelectron spectroscopy (XPS). The full wide-scan spectrum of the ZZCO samples is shown in Fig. 3a, which owns characteristic peaks for Zn, Co, O, and C elements. All the binding energies obtain in the XPS analysis are corrected for specimen charging by referencing the C 1s peak (set at 284.6 eV). The Co 2p spectrum in Fig. 3c is composed of two peaks at ∼794.7 eV for Co 2p_1/2 and ∼778.9 eV for Co 2p_3/2, determining the Co oxidation state of ZZCO samples.³ The Zn 2p spectrum is shown in Fig. 3b. The spectrum possesses two peaks at binding energies of ∼1045.2 and ∼1021.5 eV, which are attributed to Zn 2p_1/2 and Zn 2p_3/2, respectively,³ indicating the Zn(II) oxidation state of hybrid ZZCO.⁸ Significantly, another two peaks appear at binding energies of ∼1021.0 and 1044.0 eV, and can be ascribed to the divalent Zn in the ZnO structure.^8,13 More importantly, the spectrum spectra of O 1s (Fig. 3d) reveals two peaks with binding energies at ∼532.4 and ∼529.4 eV, which should be attributed to a higher number of defect sites with low oxygen co-ordination in the nano-scaled ZnCo₂O₄ and/or ZnO species^8,14 and typical lattice oxygen in the metal (Zn/Co)–oxygen framework,^8,14,15 respectively. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis shown that the molar ratio of Zn to Co is 1.3, suggesting the existence of 35 wt% ZnO and 65 wt% ZCO in the ZZCO. In consequence, the combination of XRD, TEM and XPS results can demonstrate ZnCo₂O₄ and ZnO co-exist in hybrid ZZCO.

Fig. 3 XPS spectra of the as-prepared hybrid ZZCO: (a) survey spectrum, (b) Zn 2p, (c) Co 2p, (d) O 1s.

The N₂ adsorption–desorption measurement (Fig. S4† and inset) are used to further investigate the porous structure of as-synthesized hybrid ZZCO. Fig. S4† shows the isotherm curves of hybrid ZZCO present the typical adsorption hysteresis that belongs to type IV isotherm curves, indicating the composites have a mesoporous structure with a high surface area of 34.231 m² g⁻¹ and a pore volume of 0.138 cm³ g⁻¹. Based on the BJH equation, the pore size distribution of ZZCO composites demonstrates a broad pore size distribution from 2 to 30 nm and the average pore diameter is 17.037 nm. Above all, the mesoporous architectures and high surface area would be conducive to enhancing electrochemical performance of ZZCO composites.¹⁶ Simultaneously, the abundant pore volume can buffer the volume expansion during Li⁺ insertion/extraction process.

Fig. 4a shows the first five cyclic voltammograms (CVs) of the electrode make from the ZZCO composites at a scan rate of 0.5 mV s⁻¹ between 0.001 and 3 V. One noteworthy peak can be observed at ∼0.85 V during the first cathodic scan, owing to the decomposition of ZZCO composites and the subsequent formation of a solid electrolyte interface (SEI) film, corresponding to the reduction of ZCO to Zn and Co.¹⁷ On the first anodic sweep, two oxidation peaks can be attributed to the removal of lithium and the formation of ZnO and Co₃O₄.¹⁸ Obviously, the CV profiles of the electrode from the ZZCO composites keep stable after the first cycle and do not change evidently upon further sweeps, implying highly reversible electrochemical reactions.¹⁹ According to the previous researches of ZCO, ZnO or ZZCO,^18–20 the discharge and charge processes are as follows:

ZnCo₂O₄ + 8Li⁺ + 8e⁻ → Zn + 2Co + 4Li₂O

ZnO + 2Li⁺ + 2e⁻ ↔ Zn + Li₂O

Zn + Li + e⁻ ↔ LiZn

Co + Li₂O ↔ CoO + 2Li⁺ + 2e⁻

3CoO + Li₂O ↔ Co₃O₄ + 2Li⁺ + 2e⁻

Fig. 4 (a) Cyclic voltammetry (CV) curves at a scan rate of 0.5 mV s⁻¹ in the voltage window of 0.01–3.0 V. (b) Charge–discharge curves of ZZCO at a current density of 500 mA g⁻¹. (c) Cycling performance of ZZCO and ZCO at a current of 500 mA g⁻¹. (d) Rate performance of ZZCO and ZCO at various rates.

The discharge–charge curves (Fig. 4b) of the as-prepared ZZCO electrode for the 1st, 50th, 100th and 150th cycles are examined at a current density of 500 mA g⁻¹ in the potential range from 0.001 V to 3 V. In the first discharge curve, the potential drops quickly to a 0.89 V plateau and then gradually declines to the cut-off potential of 0.001 V vs. Li/Li⁺ and this phenomenon also has appeared in the previous reports^3,21 The initial discharge and charge capacities are 869 and 554 mA h g⁻¹, respectively. The first cycle irreversible capacity loss of 32% could be due to the solid electrolyte interphase (SEI) formation and the reduction of metal oxide to metal with Li₂O formation.^3,17 After the 50th cycle onwards, the coulombic efficiency of the ZZCO electrode remained nearly 100%, and the potential plateau shifts upward to near 1.1 V (vs. Li⁺/Li) with a more sloping profile.

The cycling performance of ZZCO and ZCO electrodes are shown in the Fig. 4c. After 150 cycles at a current density of 500 mA g⁻¹, the ZZCO still exhibits a higher capacity 730 mA h g⁻¹. The gradually increasing reversible capacities during cycling are a common phenomenon for metal-oxide anode materials, maybe resulting from the gradual activation process of the metal-oxide electrodes and reversible reactions between metal particles and electrolytes.^19,20,22 By contrast, ZCO electrode shows poor cycle performance, only had ∼300 mA h g⁻¹ at 500 mA g⁻¹ after 150 cycles, demonstrating the hybrid ZZCO with hierarchical mesostructures is more beneficial to improve the high-rate performance and cycling stability.

To further investigate the high-rate performance of hybrid ZZCO, the ZCO and hybrid ZZCO electrodes are tested on rate performances. Fig. 4d shows the high-rate performance of the ZZCO and ZCO at various rates. As shown in Fig. 4d, the reversible capacities of ZZCO were ∼880, 775, 690, 500 and 240 mA h g⁻¹ at higher rates of 200, 500, 1000, 2000 and 5000 mA g⁻¹, respectively. More importantly, no obvious capacity loss happened when the current rate reduced to 500 mA h g⁻¹ again, indicating the ZZCO owned very good reversibility. Compared with hybrid ZZCO, the ZCO reveals lower reversible capacities, 600, 580, 383, 202 and 102 mA h g⁻¹ at higher rates of 200, 500, 1000, 2000 and 5000 mA g⁻¹, respectively. It is demonstrated again the hybrid ZZCO with hierarchical mesostructures can exhibit remarkable performances at high rates.

Recently, the discharge/charge capacity of the ZZCO electrode is found to be much higher than the values of pure phase ZnCo₂O₄ reported by other groups (Table 1). Hu et al.³ prepared the mesoporous ZnCo₂O₄ microspheres which showed relatively low rate capacity (580 mA h g⁻¹ at 500 mA g⁻¹) and short cycle time (40 cycles); Mohamed et al.²³ synthesized ZnCo₂O₄ nanowires possessed a relatively low reversible capacity and poor cycling stability; Rai et al.²⁴ prepared ZnCo₂O₄ nanoparticles only had ∼300 mA h g⁻¹ at 100 mA g⁻¹ after 70 cycles; Yu et al.²⁵ prepared ZnO nanoparticles and Yang et al.²⁶ synthesized mesoporous ZnO nanowires which both displayed relatively poor electrochemical performance. Some articles^27–31 showed relatively high capacity, for example, Ru et al.^27,28 prepared ZCO-CNTS and ZCO microspheres, and Xie et al.³⁰ prepared ZZCO showed relatively good electrochemical performance. By contrast, the ZZCO relatively has higher capacity and stronger cycling stability. Firstly, the excellent electrochemical performance of ZZCO electrode should be attributed to the hierarchical micro-/nanostructures with desirable mesoporosity. The novel structure could provide large electrode/electrolyte interfaces for high Li⁺ flux across and rich lithium-storage sites, reduced paths both for Li⁺ ions and electrons diffusion, leading to excellent cycle performance and stability.^12,32–34 Secondly, the typical mesoporous architectures could provide extra free volume to relieve the structural strain to some extent, thus improving the cycling ability even at high current rates.^33–35 Thirdly, the synergetic effect of the ZnO and ZCO could be one reason. High-content symbiotic ZnO in the hybrid ZZCO could be used as well-dispersed buffer domains to availably and spatially separate the coexisting highly electroactive ZCO phase at the nanoscale and further prevent the self-aggregation of nanophase ZCO during cycling,^8,36 and then effectively promoting cycling performance of the ZZCO anode.

Table 1 Compared electrochemical performance of reported Zn-based nanostructures with different morphologies

Active materials	Current density (mA g^−1)	Discharge/charge capacity (1^st cycle)	Cycle	Capacity (mA h g^−1)	Ref.
ZZCO	500	869/554	150	725	This work
ZCO-nanosheets	500	989/682	150	300	This work
ZCO-microspheres	500	1002/735	40	∼600	9
ZCO-nanowires	200	1379/1125	50	850	23
ZCO-nanoparticles	100	1112/861	70	300	24
ZCO-microspheres	100	1438/1102	100	940	31
	1000	1257/1021	1000	865
ZCO-CNTS	100	1250/1003	150	864.6	27
ZCO	100	1596.2/1236.8	120	1132	28
ZnMn2O4	500	1106/732	130	860	29
ZZCO	200	1500/1150	200	1119	30
ZnO-nanoparticles	100	1647/662	100	300	25
Mesoporous ZnO	100	1432/801	50	∼500	26

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In summary, the novel bi-component-active hierarchical ZnO/ZnCo₂O₄ nanosheets with mesostructures are prepared through a convenient and practical hydrothermal route. The hierarchical hybrid electrodes exhibit a high specific capacity of 730 mA h g⁻¹ at a current density of 500 mA g⁻¹ after 150 cycles and excellent rate capability. The excellent electrochemical performance of ZnO/ZnCo₂O₄ nanosheets results from hierarchical micro-/nanostructures with desirable mesoporosity and synergetic effect of the ZnO and ZnCo₂O₄. These novel binary oxides with hierarchical mesostructures provide a new route to design and synthesize future electrode materials for high performance LIBs applications.

Acknowledgements

This work was supported by NSFC (21271108), China US Center for Environmental Remediation and Sustainable Development, NSFC (21273118), Tianjin Municipal Science and Technology Commission (15JCZDJC40500) and MOE Innovation Team (IRT13022), National Natural Science Foundation of China (No. 21425729) in China.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, XRD pattern and SEM images of ZCO, N₂ adsorption–desorption pattern and EIS profiles of ZZCO. See DOI: 10.1039/c6ra08290d

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