MOF-Templated Synthesis of Co3O4@TiO2 Hollow Dodecahedrons for High-Storage-Density Lithium-Ion Batteries.

Co3O4 nanostructures have been extensively studied as anode materials for rechargeable lithium-ion batteries (LIBs) because of their stability and high energy density. However, several drawbacks including low electrical transport and severe volume changes over a long period of operation have limited their utilities in LIBs. Rational composite design is becoming an attractive strategy to improve the performance and stability of potential lithium-ion-battery anode materials. Here, a simple method for synthesizing hollow Co3O4@TiO2 nanostructures using metal-organic frameworks as sacrificial templates is reported. Being used as an anode material for LIBs, the resulting composite exhibits remarkable cycling performance (1057 mAh g-1 at 100 mA g-1 after 100 cycles) and good rate performance. The optimized amorphous Co3O4@TiO2 hollow dodecahedron shows a significant improvement in electrochemical performance and shows a wide prospect as an advanced anode material for LIBs in the future.

Introduction

In recent years, lithium-ion batteries (LIBs) have been widely used in smartphones, notebook computers, and hybrid electric vehicles.[1−3] However, the theoretical capacity of graphite is relatively low, about 372 mAh g–1, which is insufficient to meet the growing requirement for next-generation batteries featuring high energy density and prolonged lifespan.[4−7] Since the performance of LIBs largely depends on electrode materials, innovations in new electrode materials have attracted considerable attention.[8−10] Notably, lots of research work have been made to explore suitable anode materials with high power capacity, long cycle life, and safety.[11,12]

Transition-metal oxides (TMOs) are a class of promising candidates that are used as anode materials on account of their extensive availability, high theoretical capacity, low cost, and ease of mass production.[13−15] Nanostructured TMOs are of great importance to experimentally realizing high-performance and cost-effective anode materials, for example, nanorods,[16] nanofibers,[17] nanoplatelets,[18] and nanoflowers19 have been reported. The dynamic structure can not only adapt to the volume change during the cycle but also shorten the Li+ transportation path and improve the storage performance.20

Metal–organic frameworks (MOFs) are crystalline inorganic and organic hybrid materials featuring well-defined framework-like structures and high surface areas. The highly ordered structures of MOFs can also function as promising sacrificial templates for synthesizing porous materials.[21,22] A series of transition-metal oxides derived from MOFs have been utilized in high-performance lithium-ion batteries, such as Co3O4,[23][23] CuO,[24] CuO/Cu2O,[25] NiO,[18] NiCo2O4/NiO,[26] NiFe2O4, ZnFe2O4, and CoFe2O4,[27] and Fe3O4/VO.[28] Among these MOF-derived TMOs, CO3O4 has attracted much attention due to its good theoretical capacity (890 mAh g–1) and excellent electrical conductivity. Dan et al.20 have reported porous Co3O4 hollow tetrahedra. [Co3L2(TPT)2·xG] was selected as a precursor for the preparation of hollow Co3O4, heated at 500 °C for 4 h in air, and cooled to room temperature to get Co3O4 powder. Wang et al.[29] have proposed the reversible capacity of Co3O4/TiO2 hollow composites at 500 mA g–1 after 200 cycles to be 642 mA h g–1, maintaining 96.9% of the initial capacity. Unfortunately, their cycling performance was severely affected during lithiation/delithiation processes.

Herein, we report a novel strategy for the preparation of hollow Co3O4@TiO2 composites with dodecahedron structures via thermal decomposition of ZIF-67 and TiO2 coating. Moreover, their unique porous Co3O4@TiO2 hollow amorphous structure not only provides fast ion-conducting channels but also has sufficient buffer space for volumetric expansion during the cycling process. Since the capacity of Co3O4 is high, the battery capacity usually declines quickly because of large volume change during the charging/recharging cycle. Furthermore, TiO2 has a very low capacity but a high stability, long cycle life, low cost, and environmentally benign character.[22,30−33] When the anode materials of the LIBs are evaluated at the same time, their strengths and weaknesses are complementary; there is also a gradation of the porous structure, making the anode have increased specific capacity, rate capability, and cycling stability.

Material Characterization

Field-emission scanning electron microscopy (FESEM, ZEISS MERLIN, Germany) with an energy-dispersive spectrometer (EDS), TEM, and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100) were used to observe the morphologies, elements, and structures of ZIF-67, Co3O4 and Co3O4@TiO2. X-ray powder diffraction (XRD) patterns were detected on Bruker D8 equipped with Cu Kα radiation (λ = 1.5406 Ǻ). Thermogravimetric analysis (TGA) was performed using a PerkinElmer thermogravimetric analyzer under an air atmosphere from room temperature to 800 °C at a heating rate of 20 °C min–1. X-ray photoelectron spectroscopy (XPS) was analyzed on an AXIS165 photoelectron spectrometer using an AlK scalar X-ray source. N2 adsorption/desorption measurements (BET, NOVA1000e) were performed to understand the pore properties of samples.

Electrochemical Measurements

The electrochemical behavior of the Co3O4@TiO2 hollow structure was investigated by using CR 2016 coin cells with pure lithium foil as the counter electrode, Celgard 2400 as the separator, 1 M LiPF6 was dissolved in ethylene carbonate/propylene carbonate (1:1, w/w) as the electrolyte. The active materials prepared (70 wt %), carbon black (20 wt %), and binder poly(vinylidene fluoride) (10 wt %) were mix with in NMP to form a homogeneous slurry, which was coated on a copper sheet and finally dried in a vacuum oven for 24 h. Galvanostatic charge/discharge measurements were carried out on a LAND CT2001A battery-test system under different current densities within the voltage range of 0.01–3.0 V. The cyclic voltammogram (CV) was performed on a CHI660C Electrochemical Workstation at a sweep rate of 0.1 mV s–1 in a potential range of 0.01–3.0 V vs Li/Li+. Electrochemical impedance spectra (EIS) was tested on the CHI660E within a frequency range of 100 kHz to 10 mHz.

Results and Discussion

The synthesis process of the Co3O4@TiO2 amorphous structure is displayed in Scheme . The Co3O4 hollow structure was synthesized for the first time via ZIF-67 as a precursor (heated to 450 °C at a speed of 2 °C min–1 in the air). Whereafter, by means of chemical deposition, a layer of amorphous TiO2 was fully coated over the Co3O4 to form amorphous Co3O4@TiO2, which was then subjected to annealing heat treatment at 400 °C at a speed of 2 °C min–1 in an argon atmosphere and kept for 5 h, which was converted into an amorphous hollow dodecahedron structure.

Scheme 1

Schematic Illustration of the Procedure Used to Fabricate Amorphous Co3O4@TiO2 Composite Hollow Dodecahedrons

The crystalline structure and phase composition of the as-synthesized sample ZIF-67 were characterized by powder X-ray diffraction (PXRD) measurement, as shown in Figure a. All of the diffraction peaks of the prepared precursor ZIF-67 are consistent with previous reports.[34] The microstructure of the as-synthesized ZIF-67 was investigated by FESEM, and the representative image is shown in Figure b,c. The morphology of the sample is a dodecahedron microcrystal with a smooth surface and sharp edges, with a diameter of 0.8–1 μm. The simulation model image is displayed in the illustration (Figure c). The N2 adsorption/desorption isotherm of ZIF-67 is shown in Figure S1, Supporting Information, which gives the specific surface area (1005 m2 g–1). The TGA curve of the synthesized ZIF-67 precursor is shown in Figure d. The mass loss starts at about 320 °C, and there is no significant mass loss after 440 °C. The total mass loss is approximately 62%, affording a dark Co3O4 powder. Therefore, by setting the calcination temperature to 450 °C, we can obtain smaller and stable crystals.

Figure 1

(a) Experimental and simulated XRD patterns of ZIF-67. (b) Low- and (c) high-magnification FESEM images of ZIF-67. (d) TGA curves of ZIF-67 under an air atmosphere.

The X-ray diffraction (XRD) patterns of Co3O4 obtained by pyrolyzing ZIF-67 and the Co3O4 prepared by the solvothermal method were collected on a powder X-ray diffractometer (Rigaku, SmartLab) with the u–2u scan mode from 25 to 70°, as shown in Figure a. Co3O4 hollow polyhedrons were determined to be cubic cobalt oxide, as evidenced by the characteristic diffraction peaks, which are in good agreement with a cubic spinel Co3O4 (JCPDS card no. 42-1467).[35] However, the characteristic diffraction peak of as-prepared Co3O4 is consistent with the standard card but the crystallinity is not ideal. The PXRD pattern of the as-synthesized Co3O4@TiO2 is presented in Figure b, showing the diffraction peaks of Co3O4 and TiO2 (JCPDS card no. 21-1272). However, after coating with TiO2 and annealing again, the diffraction peaks of Co3O4@TiO2 are similar to Co3O4, indicating that the coating process of TiO2 did not affect the crystal structure. However, we could not obtain the characteristic XRD pattern attributed to TiO2 presumably due to its low content amorphous form.[36] To further characterize the structure of Co3O4@TiO2, energy mapping was performed and the data are shown (Figure S2a). These results confirmed the presence of Co, O, and Ti, suggesting that we have obtained a composite material. EDS spectra illustrate that the atomic ratio of Co/Ti/O in the Co3O4@TiO2 nanocomposite is 0.37:0.04:0.59, as shown in Figure S2b. The XPS spectra are shown in Figure a–c. The characteristic peaks of Co, Ti, and O chemical elements are detected, confirming that only these three elements are present in the composite. The two peak values of 780.6 (Co 2p3/2) and 796.3 eV (Co 2p1/2) are obvious, in accordance with Co2+ species. In the XPS spectrum of Ti 2p, it can be observed that the peak values of Ti 2p1/2 and Ti 2p3/2 are 463.7 eV and 457.9, which proves the existence of Ti4+. To know the ratio of Co and Ti metal ions more accurately, we have carried out the ICP-MS test with Co3O4@TiO2 (Table ). Prior to ICP-MS measurements, Co3O4@TiO2 was washed and centrifuged with 30 mL of deionized water, then with 30 mL of EtOH, and then dried under vacuum for 12 h. Co3O4@TiO2 with a weight of 0.1 mg was prepared and dissolved in 37.5% hydrochloric acid at room temperature. The sample was then diluted to 10× in hydrochloric acid (2.5 mL) and deionized water. A PerkinElmer Optima 8000 ICP-MS was used for further analysis. The resulting calibration curves have a minimum R2 of 0.999. The individual results of the samples were averaged to determine the metal ratios. ICP-MS measurements of Co3O4@TiO2 with the Co/Ti metal ratios of 6:1.

Figure 2

(a) XRD pattern of the hollow Co3O4 and as-prepared Co3O4 nanocomposite. (b) XRD pattern of amorphous Co3O4@TiO2.

Figure 3

(a) XPS survey spectrum and (b, c) high-resolution XPS spectrum of Co 2p and Ti 2p of Co3O4@TiO2.

Table 1

ICP-MS Data of Co and Ti Metal Ions in Co3O4@TiO2

name	solvent	first metal	ppm (mg L–1)	second metal	ppm (mg L–1)
Co3O4@TiO2	HCl	Co (228.616)	1.332	Ti (334.940)	0.191

In addition, the morphology was completely maintained after calcination: the obtained Co3O4 had a rhombic dodecahedral shape (Figure a). Compared to the precursor, the Co3O4 rhombic dodecahedral structure shrinks by 10% in size and its structural diameter is about 800 nm. The microstructure of Co3O4@TiO2 is further examined by TEM analysis. Figure b,c reveals that there is a distinct electron distribution difference between the melanocratic edge and light-colored center, confirming the amorphous structure of the nanocomposites. The HRTEM image of Co3O4@TiO2 shows that the thickness of the TiO2 coating is about 15 nm. To demonstrate the high structural stability of the Co3O4@TiO2 material (Figure S3), we also tested the FESEM of the material after 10 cycles at a current density of 0.1C. Found after several lithiation processes and after the delithiation process, the material still retains the original frame structure. It is proved that the structure of Co3O4@TiO2 is stable.

Figure 4

(a) FESEM images of Co3O4. (b, c) TEM images of amorphous Co3O4@TiO2.

To gain further insight into the porous hollow rhombic dodecahedral Co3O4 and Co3O4@TiO2, Brunauer–Emmett–Teller (BET) analysis was used to quantify the specific surface area and pore size distribution. As shown in Figure S4a,b, Co3O4@TiO2 shows a typical IV curve with a hysteresis loop, indicating its mesoporous character. Co3O4@TiO2 exhibits a broad pore size distribution, and the first-rate BET specific surface area was 65.350 m2 g–1. Correspondingly, the specific surface area of Co3O4 is 44.369 m2 g–1, less than that of Co3O4@TiO2. In addition, the average pore size of Co3O4@TiO2 (28.197 nm) is larger than that of Co3O4 (15.312 nm), which is beneficial for enhancing the transport of Li+ across such a matrix and adjusting the volume change in the discharge/charge process.

The uniqueness of the aforementioned mesoporous Co3O4@TiO2 composite is that it not only provides electron and ion-conducting channels but also enables sufficient buffer space for volumetric expansion of the material during the charge/discharge cycling process.[26]

Electrochemical Performance

To evaluate the potential application of the Co3O4@TiO2 composite material as a novel energy storage material for the anode electrode of LIBs, cyclic voltammetry (CV) and galvanostatic charge/discharge tests were performed. Figure a shows the Co3O4@TiO2 electrode between the voltage range of 0.001 and 3.0 V at a scan rate of 0.1 mV s–1 for the first three cycles. The electrochemical reactions of Co3O4 and TiO2 with lithium are described as followsIn the first cathodic scan, it is possible that the peak near 0.55 V corresponds to the reduction of Co2+ to metallic Co, and the formation of a solid electrolyte interface (SEI) layer and amorphous Li2O. In the subsequent anodic scan, a wide peak located at 2.15 V vs Li/Li+ can be attributed to the reoxidization of metallic cobalt to Co2+ and the decomposition of Li2O. The characteristic cathodic peaks at 1.35 and 1.42 V and the anodic peak at 2.1 V are due to the insertion/extraction of lithium ions (reaction ). The wide anodic peak at 2.1 V and the cathodic peak at 0.9 V are related to the reversible oxidation/reduction of Co/Co3O4 (reaction ). However, after the second cycle, the CV curves remain nearly identical during cycling, indicating that Co3O4@TiO2 exhibits excellent cycling stability and rate retention.

Figure 5

(a) CV curves at a scan rate of 0.1 mV s–1 in the voltage range of 0.01–3.0 V. (b) Charge/discharge profiles at a current density of 0.1 A g–1. (c) Cycling performance at a current density of 0.1 A g–1. (d) Rate capabilities of Co3O4, hollow Co3O4, and Co3O4@TiO2.

Representative charge–discharge voltage profiles for the 1st, 5th, and 10th cycles of the Co3O4@TiO2 electrode at a current density of 100 mA g–1 (about 0.1C) are shown in Figure b. In the first cycle, there is a discharge voltage plateau at about 1 V due to the heterogeneous reaction mechanism between metallic Li and the electrode. The initial discharge and charge capacities are 1430.7 and 1102.0 mAh g–1, respectively. In the initial cycle, the irreversible capacity loss is caused by electrolyte decomposition and the formation of a SEI film, with a capacity loss of approximately 23%.[37] The 5th and 10th discharge capacities of the Co3O4@TiO2 electrode are 1312.39 and 1326.792 mAh g–1, respectively. The discharge/charge profiles of 5th and 10th almost overlap with each other, indicating that the electrochemical performance of the anode is stable.

Figure c shows the cycling stability of the Co3O4@TiO2 electrode, hollow Co3O4, and as-prepared Co3O4 at a current density of 100 mA g–1. The initial discharge capacity of the LIB of Co3O4@TiO2 is 1430 mAh g–1, which is 35% higher than the LIB based on solely Co3O4. At the same time, the amorphous type Co3O4@TiO2 composite electrode exhibited good cycling stability. After cycling 100 times at 100 mA g–1, the capacity was approximately 1057 mAh g–1, accompanied by an average Coulombic efficiency of over 101%. Note that the average Coulombic efficiency of hollow Co3O4 and as-prepared Co3O4 is also above 99%, indicating that the conversion reaction between Li and Co3O4 is highly reversible. On the contrary, the capacity of the Co3O4 anode dropped to 708 mAh g–1 after 100 cycles and continued to decrease to 32%. The two electrode discharge capacities gradually increased around the first 10 cycles, which further confirmed the presence of electrode activation. For TMO electrode materials, the increased capacity in the early cycling can be explained by the following factors: (1) the polymer layer due to electrolyte degradation contributes some extra capacity through “pseudocapacitative behavior”;[38] (2) the diffusion kinetics of Li+ ions can be improved;[39,40] and (3) lithium interface storage also provides some capacity.[41] For as-prepared Co3O4, from the 1st cycle to the 13th cycle, the capacity dropped rapidly and then slowed down to about 53 cycles and the capacity dropped to 372 mAh g–1, which is the same as that of graphite. This indicates that the volume of the common Co3O4 material changes significantly in the charging/discharging process, and the capacity retention rate is too low. Amorphous type Co3O4@TiO2 nanostructures have larger capacities than pristine hollow Co3O4 and common Co3O4, and the reversible capacity of amorphous type Co3O4@TiO2 nanostructures is much higher than those of most previous reports on MOF-derived Co3O4 samples. Our sample was compared with the previously published Co3O4 anode material extracted from MOFs for lithium-ion batteries in terms of morphology and cycling performance (Table ). In addition, Figure d shows the cycling stability of the as-prepared Co3O4 electrode, the Co3O4 electrode and the Co3O4@TiO2 electrode at the current density range of 0.1–4 A g–1. With the current density being increased gradually from 0.1 to 0.2, 0.4, 1, and 2 A g–1, the specific capacity of Co3O4@TiO2 decreased from 1291 to 1083, 1030, 736, and 300 mAh g–1. However, the capacity of hollow Co3O4 is reduced from 733 to 619, 416, 236, and 118 mAh g–1, and the capacity of common Co3O4 degrades from 452 to 335, 177, 79, and 44 mA g–1. The reversible capacity of Co3O4@TiO2 remains at 60 mAh g–1 at a high current density of 4 A g–1, which is 46% greater than that of hollow Co3O4 and 87% better than that of as-prepared Co3O4. More importantly, after a high-rate discharge–charge cycle, the electrode capacity could be restored despite the decrease in current density. When the current density charged to 100 mA g–1, its discharge capacity restored to 1 Ah g–1, and the Co3O4 capacity restored to 500 mAh g–1. However, the as-prepared Co3O4 capacity was only restored to 242 mAh g–1. The high reversible capacity, good cycling performance, and rate capability of the Co3O4@TiO2 electrode can be ascribed to the following reasons: the hollow porous structure of Co3O4@TiO2 can furnish enough space to adapt the volume changes and to reduce crushing. Meanwhile, the repetition of the lithiation/delithiation process provides more electroactive sites within the pores for rapid electrochemical reactions and facilitates the delivery of lithium ions at the electrolyte/electrode interface, leading to a significant improvement in performance.

Table 2

Comparison of the Capacity of the Present Work with Co3O4@TiO2

sample	morphology	current density (mA g–1)	capability (mAh g–1)/cycles	references
Co3O4@TiO2 (this work)	amorphous	100	1057/100
Co3O4	hollow tetrahedron	200	1052/60	(48)
Co3O4-a	hierarchical structures (twin hemispherical)	100	470.3/90	(47)
Co3O4-b	hierarchical structures (novel flower-like)	100	529.2/90	(47)
Co3O4	cuboids	100	886/60	(43)
NiO–Co3O4@C	yolk@shell	100	803/100	(44)
NiO–Co3O4@C	concave cubic	100	870/100	(44)
Co3O4	porous hollow microfibers	100	787.6/200	(45)
Co3O4	hollow dodecahedron	100	780/100	(23)
Co3O4	hollow nanosphere	100	820/100	(46)

We then employed electrochemical impedance spectroscopy (EIS) to further study the lithium storage performance of LIBs containing Co3O4@TiO2, hollow Co3O4, and Co3O4. The EIS spectra were compared at ambient temperature in the frequency domain of 100k to 0.01 Hz. In the Nyquist diagram, R (Ω) represents the cell’s Ohmic resistors and the electrode/electrolyte interface (Rct) represents the charge transfer resistor. All of the electrodes of the Nyquist plot including a depression of the semicircle show a subsequent slash in the low-frequency range and in the mid-frequency range, which is attributed to the charge transfer impedance at the electrode/electrolyte interface (Rct) and the electrolyte resistance (Rs).[42] According to the EIS analysis (Figure ), the as-prepared Co3O4 had the largest semicircle diameter and its charge transfer resistance was about 300 Ω. The Co3O4@TiO2 amorphous composite material has a smaller semicircle diameter than hollow Co3O4, and its charge transfer resistance was about 210 Ω. After the combination of TiO2, the charge transfer resistance decreased to 100 Ω, suggesting that the Co3O4@TiO2 electrode is indeed a promising anode, featuring desirable material properties including lower contact and charge transfer impedance.

Figure 6

Nyquist plots of Co3O4, hollow Co3O4, and Co3O4@TiO2 in the frequency range between 100 kHz and 100 mHz.

Conclusions

In summary, a new templated synthesis of high-performance hybrid materials for LIBs is proposed. Amorphous hollow dodecahedron nanostructures composed of TiO2-encapsulated Co3O4 are synthesized by utilizing ZIF-67 polyhedrons as sacrificial templates by pyrolysis in air to afford Co3O4 with a coating of TiO2 in the outer layer. It is then annealed in an Ar atmosphere to form an amorphous Co3O4@TiO2 composite exhibiting excellent electrochemical performance, good rate performance, and excellent cycling stability. In addition, MOF-templated synthesis of metal oxides is an effective technology for the preparation of porous nanomaterials with a well-designed structure and is expected to be used for the production of high-performance anode materials for advanced LIBs.

Experimental Section

Synthesis of ZIF-67

2-Methylimidazole (1.31 g, 14.8 mmol) and Co(NO3)2·6H2O (1.164 g, 3.9 mmol) were dissolved in 25 mL of methanol and 25 mL of ethanol, respectively. After sonication for 10 min, the solutions were mixed and were allowed to stand at room temperature for 24 h. The resultant purple precipitate was collected by centrifugation, then washed with ethanol several times, and dried at 80 °C.

Synthesis of Co3O4

ZIF-67 was placed in a tube furnace and then heated to 450 °C at a rate of 2 °C min–1 in the air and maintained for 2 h. Finally, its color changed from purple to dark color.

Synthesis of As-Prepared Co3O4

For this, 0.50 g of Co(CH3COO)2·4H2O was dissolved in 25.0 mL of water, and 2.5 mL of 25% ammonia was added to it slowly under intense agitation. Then, the mixture was stirred in air for about 10 min to form a uniform slurry. Then, the mixture was transferred to an autoclave, sealed, and kept at 150 °C for 3 h. After that, the autoclave is cooled down naturally to room temperature. Then, the product was washed with water three times and put into a vacuum drying oven at 60 °C for 4 h.

Synthesis of Co3O4@TiO2

Co3O4 (0.2 g) was dispersed in HCl (0.1 M, 95.725 mL), and then TALH (4.275 mL; 50 wt % aqueous solution) was added to it. The dispersion was stirred at 25 °C for 3 h. The resulting dark particles were harvested by centrifugation and washed several times with deionized water and ethanol, respectively, and then dried overnight at 70 °C. This dried sample was heated to 400 °C in a quartz tube furnace at a speed of 2 °C min–1 in an argon atmosphere and kept for 5 h. Finally, the Co3O4@TiO2 core–shell structure was collected.