MOF-Derived Hetero-Zn/Co Hollow Core-Shell TMOs as Anode for Lithium-Ion Batteries

Abstract

In this work, metal–organic frameworks (MOFs) were used as precursors to prepare Zn/Co oxide with a porous dodecahedral core-shell structure. Herein, a low-temperature self-assembly calcination and hydrothermal strategy of imidazole-based Zn-Co-MOF was used. As anode of lithium-ion batteries (LIBs), ZnO/Co₃O₄ has good cycling stability, the specific discharge capacity of ZnO/Co₃O₄ is stable at about 640 mAh g⁻¹ after 200 cycles, and its coulombic efficiency (CE) is stable above 95% after the first 20 cycles. When the current density is 0.6 A/g, the discharge capacity is 420 mAh g⁻¹. This excellent electrochemical performance is attributed to its unique porous hollow structure and unique heterojunction electrode interface, which improves the Li⁺ storage capacity, increases the contact area between the electrode and the electrolyte, and improves the overall electrochemical activity. In addition, the synergistic effect of ZnO and Co₃O₄ also plays an important role in improving the electrochemical performance.

1. Introduction

LIBs have significant advantages such as light weight, small size, high specific energy, small self-discharge, no memory effect, and good high-current discharge performance [1,2], whereas conventional graphite anodes are limited by their low theoretical capacity (372 mAh g⁻¹) and safety concerns related to dendrite formation issues [3]. Therefore, finding high-performance anode materials is crucial for developing next-generation LIBs with higher energy density and high safety.

Transition metals/metal compounds (such as Co₃O₄ [4], TiO₂ [5], and ZnS [6], among others) (TMOs) are considered as one of the most promising candidates thanks to their higher theoretical capacities. However, TMOs have inherent disadvantages such as large volume change and poor electrical conductivity [7]. To overcome the about-mentioned weak points, MOFs are introduced as precursors for the synthesis of multi-metal oxides [8]. For example, Li’s group reported an egg-shell ZnO/NiO microsphere based on Zn-Ni MOFs with a specific capacity of 1008.6 mAh g⁻¹ after 200 cycles at 0.1 A/g [9]. The biphasic Co₃O₄/Fe₃O₄ hollow nanospheres prepared from Fe@Co-MOFs exhibited a specific capacity of 937 mAh g⁻¹ after 150 cycles at 0.1 A/g, and their structures remained in good condition after 150 cycles [10]. The porous hollow nanostructures of MOF-derived material can provide 3D interconnected channels for the rapid transfer of Li⁺, increase the contact area between the electrode and the electrolyte, as well as provide additional internal space for volume change [11,12,13,14]. Based on the above studies, it is feasible and promising to synthesize multi-metal TMOs with regular shapes using MOFs as precursors.

This paper demonstrates a synthetic strategy for Zn/Co TMOs anode with porous core-shell nanocages. The synthesis process involves a low-temperature hydrothermal and calcination strategy. As a result of this strategy’s simplicity, adaptability, and low cost, mass production prospects are quite bright. Furthermore, the Zn and Co atoms are evenly distributed throughout the crystal structure and separated by organic linkers, which make it easier for ultrafine metal oxide nanocomposites to form during the calcination process in air.

The particular structure and distinctive electrode interface heterostructure of ZnO and Co₃O₄ provide this composite with a high electrochemical performance when tested as an anode for LIBs.

2. Experimental

2.1. Preparation of Zn-Co-MOF

Typically, C₄H₆O₄·Zn·2H₂O (3.28 mmol) and 2-methylimidazole (3.65 mmol) were added to 40 mL of deionized water and the solution was stirred and allowed to stand at room temperature for 24 h. The precipitate obtained by centrifugation was washed several times with methanol and deionized water and dried at 80 °C for 12 h to obtain Zn-MOF. Then, 0.08 g of Zn-MOF was weighed and dissolved in 10 mL of methanol to obtain a Zn-MOF solution. Then, Co(NO₃)₂·6H₂O (0.74 mmol) and 2-methylimidazole (10.9 mmol) were then added to 6 mL of methanol and stirred for 5 min. Finally, the above solution was added to the Zn-MOF solution, stirred evenly, and reacted in the reactor at 120 °C for 4 h. The pellet obtained after centrifugation was washed several times with absolute ethanol and dried to obtain Zn-Co-MOF.

2.2. Preparation of ZnO/Co₃O₄

The Zn-Co-MOF precursor sample was annealed in a furnace with Ar atmosphere to 300 °C for 2 h with a heat-treatment rate of 2 °C min⁻¹. Then, the obtained sample was stored in air at 300 °C for 1 h, and finally the ZnO/Co₃O₄ composite was obtained.

2.3. Material Characterization Test

An X-ray diffractometer (XRD, Bruker D8 Advance, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.54056 Å) was used to analyze the sample crystals. The morphology of the sample was characterized by scanning electron microscope (SEM, Quanta200, Eindhoven, The Netherlands) and transmission electron microscope (TEM, JEOL 2001F, Tokyo, Japan).

2.4. Electrochemical Test

Dispersion of the TMOs (active material: 1 mg), PVDF, and acetylene black in the N,N-Dimethylpyrrolidone solution took place at a weight ratio of 8:1:1. After stirring, a slurry of the material was applied on the copper foil with a radius of 0.5 cm and dried in a vacuum oven at 60 °C for 6 h. The button battery (CR2032) was assembled in the glove box with the microporous polypropylene film as a separator, a metallic lithium foil as the counter electrode, and 1 M LiPF₆ in ethylene carbonate (EC)–dimethyl carbonate (DMC)–ethylene methyl carbonate (EMC) with a volume ratio of 1:1:1 as the lithium-ion electrolyte. Finally, the Wuhan Lan Dian battery tester (BTRBTS CT2001A, Wuhan, China) was used to conduct constant current charge and discharge test of the battery in the voltage range of 0.01–3.00 V. The CHI608E electrochemical workstation was used to perform the volt-ampere cycle test (voltage range: 0.01–3.00 V, scan rate: 0.1 mV s⁻¹) and electrochemical impedance spectroscopy test.

3. Results and Discussion

3.1. Analysis and Characterization

Figure 1a–c are the SEM images of Zn-Co-MOF. It can be clearly seen that the Zn-Co-MOFs precursors are uniformly distributed and the surface is very smooth. The precursor particles are rhombic polyhedrons with regular morphology. Besides, it can be observed that ZnO/Co₃O₄ (Figure 1d–f) presents a dodecahedral structure with an edge length of about 1.5 μm. This specific hollow structure enhances the storage capacity of Li⁺ [15,16,17]. During the growth and transformation of Zn-Co-MOF to ZnO/Co₃O₄, Zn-Co-MOF is self-assembled by internal metal ion centers and external imidazole ligands. During the low-temperature calcination process, the outer Co-MOFs shell is squeezed, the inner hollow section increases, and the Zn-MOFs particles are further decomposed. The surface of the particles is smooth with varying degrees of depressions and wrinkles at the boundaries, which can be seen from the scanned images.

The size and morphology of the ZnO/Co₃O₄ nanocomposite were further investigated by TEM. Figure 2a,b can clearly show that ZnO/Co₃O₄ presents a hollow dodecahedron porous core-shell structure. Figure 2c,d show the HRTEM of ZnO/Co₃O₄, where it can be seen that the material is crystalline. The crystal planes of ZnO are (100) and (002) and the measured lattice spacings are 0.28 nm and 0.27 nm, respectively. The SAED pattern of ZnO/Co₃O₄ (Figure 2c illustration) indicated the presence of ZnO and Co₃O₄, representing the predominant growth of ZnO and Co₃O₄ crystal planes, which is consistent with the XRD data.

Using X-ray diffraction (XRD), the crystal structure and phase analysis of the synthesized samples are performed. The red-star diffraction peaks represent the typical patterns of the spinel Co₃O₄’s face-centered-cubic phase. It can be seen from Figure 3 that the main diffraction peaks of Co₃O₄ in the ZnO/Co₃O₄ material are at 2θ of 18.975°, 36.845°, 44.762°, 59.271°, and 65.219°, corresponding to crystal planes of (111), (311), (400), and (511), respectively, with no impurity diffraction peaks (JCPDS Card No.43-1003). Besides, those marked with a green star can be indexed to the hexagonal ZnO (JCPDS Card No.36-1451). The diffraction peaks appear at the following positions: 2θ = 31.370°, 34.534°, 47.585°, and 56.711°, corresponding to the (100), (002), (102), and (110) crystal planes of ZnO, respectively.

3.2. Electrochemical Performance

It can be clearly seen from Figure 4 that, during the initial cycle of discharge, two distinct reduction peaks appear at 0.35 V and 0.7 V. The larger reduction peak at 0.7 V corresponds to the multi-step reduction of cobalt ions to Co. The smaller reduction peak at 0.35 V corresponds to the reduction of Zn²⁺ to Zn during the discharge process. Meanwhile, the reduction of Zn²⁺ to Zn is accompanied by the formation of a solid electrolyte interface film (SEI film) and the process is irreversible. During the initial cycle of charging, a relatively obvious oxidation peak appeared at 2.2 V and a very small oxidation peak appeared at 1.6 V. The oxidation peak at 2.2 V corresponds to the formation process of cobalt ions and the oxidation peak at 1.6 V corresponds to the oxidation of Zn to Zn²⁺. Compared with the initial cycle, the amplitudes of the oxidation and reduction peaks in the second cycle were significantly smaller, as well as the reduction peak shift to the right to a certain extent; the integral area was also significantly smaller than that in the initial cycle. This is mainly due to the formation of the SEI film, which leads to an irreversible decrease in the capacity of the battery [18,19,20]. During the second cycle of discharge, there is a decreased reduction peak at 0.78 V. In the following charge–discharge cycles, the amplitude of the reduction peak gradually became smaller and stabilized owing to the reconstruction of the SEI film and the consumption of lithium ions. This mainly correspond to the reduction of Zn²⁺ to Zn, accompanied by the reduction of Co²⁺ to Co, while the reduction peaks of cobalt ions in other valence states to cobalt are insignificant. The main reason for the reduction in the number of oxidation peaks and reduction peaks is that, in the process of calcination, when Co forms ions, cobalt elements in various valence states will be formed. Most of the valence states of Co ions are Co²⁺, so the redox peaks of Co ions in other valence states are insignificant during the charging process of the initial cycle and the charge-discharge process after the second cycle. The redox process of the third circle is similar to the redox process of the second circle. Both the oxidation peak and the reduction peak are close to those of the second circle, and the integral area does not decrease greatly. Therefore, the electrochemical reversibility of LIBs assembled with ZnO/Co₃O₄ is high.

Figure 5 depicts the ZnO/Co₃O₄ electrode’s AC impedance spectrum. It is clear that it comprises a linear low-frequency region and an unmistakable semi-circular high-frequency portion. Particularly, the semicircle is associated with the charge transfer resistance at the electrode/electrolyte interface, while the intercept stands in for the ohmic resistance. The inset shows the equivalent circuit diagram fitted by Z-view software. The parameters of the fitting circuit components are shown in

Table 1. Circuit element fitting parameters.

Element	Value	Error	Error%
Rs	3.615	0.11065	3.0609
CPE1-T	2.7689 × 10−5	2.195 × 10−6	7.9273
CPE1-P	0.6874	0.0072492	1.0546
RCT	157	8.5552	5.4492
W1	56.36	25.91	45.972

.

Figure 6 shows the discharge-charge voltage distributions of ZnO/Co₃O₄ at the 1st, 10th, and 20th cycles at a current density of 0.1 A g⁻¹ between 0.01 and 3.0 V. The first discharge specific capacity is about 1250 mAh g⁻¹, the charge specific capacity is about 880 mAh g⁻¹, and the corresponding initial CE is 70.4%. The relatively low CE can be attributed to the irreversible capacity loss, including decomposition of electrolyte, formation of SEI film, and some undecomposed phases, which are common for most anode materials. In the following cycles, the specific capacity decays rapidly, and the discharge specific capacity is about 740 mAh g⁻¹ and the charge specific capacity is about 700 mAh g⁻¹ in the second cycle; its coulombic efficiency rapidly increases to 94.5%. ZnO/Co₃O₄ has a voltage plateau around 2.5 V for the first charge and a voltage plateau around 0.7 V for the first discharge, which is consistent with the cyclic voltammetry (CV) curve.

Figure 7 shows the cycling performance of ZnO/Co₃O₄ at a current density of 0.1 A g⁻¹. The initial discharge specific capacity reaches 1600 mAh g⁻¹ and, after several cycles, the discharge specific capacity is stable at about 640 mAh g⁻¹. This is because of the increased electrochemical access to the active material that occurs as cycling proceeds and large surface area provided by the hollow dodecahedral 3D core-shell structure of ZnO/Co₃O₄. The coulombic efficiency remains stabilized above 95% after multiple cycles.

In order to maintain the consistency of the testing electrode samples, thereby gaining insight into the durability of the electrode, the rate performance test was carried out after 200 cycles of the battery.

Figure 8 shows the rate capability of ZnO/Co₃O₄. It can be seen that its specific discharge capacity tends to be stable at high current densities, which again confirms that the hollow dodecahedral porous core-shell structure still has good performance at high current densities. When the current density is 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 A/g, the specific capacity is 605, 582, 524, 513, 480, and 420 mAh/g respectively. When the current density drops to 0.1 A/g, the specific capacity can be recovered to 590 mAh/g, further indicating that the structure of the electrode material is stable and exhibits excellent rate performance.

According to the above-mentioned discussions, ZnO/Co₃O₄ exhibits good rate performance and cycling stability, which are the most critical requirements for LIBs. For comparison, the morphological structures and electrochemical properties of the single metal oxides are listed in

Table 2. Morphology and electrochemical properties of relevant metal oxides.

Samples	Morphology and Structure	Initial Specific Capacity	Cycle Performance	Rate Performance
Co3O4	spherical	1113 mAh g−1	150 mAh g−1 after 50 cycles	162 mAh g−1 at 400 mA/g
ZnO	cylindrical shaped nanorods	1226 mAh g−1	230 mAh g−1 after 100 cycles	320 mAh g−1 at 300 mA/g
ZnO/Co3O4	dodecahedral structure	1250 mAh g−1	640 mAh g−1 after 100 cycles	513 mAh g−1 at 400 mA/g

. Some single metal oxides such as ZnO [21] and Co₃O₄ [22] have high specific capacity, but their rate performance and cycling performance are poor.

The excellent electrochemical performance can be attributed to the unique structure feature of porous hollow core-shell nanocages and Zn/Co heterogeneous interface, which can effectively improve the electrochemical kinetics, shorten the transport pathways of Li⁺ and electrons, and reduce the diffusion length and resistance for the transport of Li⁺ and electrolyte molecules. Finally, the synergetic effect of Co₃O₄ and ZnO may also contribute to the excellent electrochemical performance. Compared with the relevant reported papers, this paper reports a creative method combining low-temperature calcination (300 °C) and low-temperature hydrothermal (120 °C) strategy to realize the preparation of complex hollow nanocubic composite metal oxides. It provides a new strategy for subsequent novel energy storage multi-metal TMOs electrodes and catalytic materials.

4. Conclusions

In conclusion, we have successfully synthesized ZnO/Co₃O₄ TMOs with a porous dodecahedral structure by low-temperature calcination and hydrothermal strategy. Owing to the characteristics of the specific hollow porous core-shell nanocages structure and Zn/Co heterogeneous interface, ZnO/Co₃O₄ exhibits good cycling performance and high reversibility compared with the TMOs of ZnO or Co₃O₄. When used as anode for LIBs, ZnO/Co₃O₄ nanocomposites exhibit a high reversible capacity of 640 mAh g⁻¹ after 100 cycles at a current density of 100 mA g⁻¹. Its coulombic efficiency (CE) is stable above 95% after the first 20 cycles. This work may also provide a general method to design and prepare heterogeneous multi-metal oxides for extensive applications in high-performance LIBs, supercapacitors, and catalysis.