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. 2020 Aug 19;5(34):21488–21496. doi: 10.1021/acsomega.0c02037

Nanoporous Composites of CoOx Quantum Dots and ZIF-Derived Carbon as High-Performance Anodes for Lithium-Ion Batteries

Dujiang Lu 1, Chao Yuan 1, Mengchun Yu 1, Yinghui Yang 1, Chao Wang 1, Rongzhang Guan 1, Xiufang Bian 1,*
PMCID: PMC7469398  PMID: 32905499

Abstract

graphic file with name ao0c02037_0008.jpg

Transition-metal oxides are attracting considerable attention as anodes for lithium-ion batteries because of their high reversible capacities. However, the drastic volume change and inferior electrical conductivity greatly retard their widespread applications in lithium-ion batteries. Herein, three-dimensional nanoporous composites of CoOx (CoO and Co3O4) quantum dots and zeolitic imidazolate framework-67-derived carbon are fabricated by a precipitation method. The carbon prepared by carbonization of zeolitic imidazolate framework-67 can greatly enhance the electrical conductivity of the composite anodes. CoOx quantum dots anchored firmly on zeolitic imidazolate framework-67-derived carbon can effectively inhibit the aggregation and volume change of CoOx quantum dots during lithiation/delithiation processes. The nanoporous structure can shorten the ion diffusion paths and maintain the structural integrity upon cycling. Meanwhile, kinetics analysis reveals that a capacitance mechanism dominates the lithium storage capacity, which can greatly enhance the electrochemical performance. The composite anodes show a high discharge capacity of 1873 mAh g–1 after 200 cycles at 200 mA g–1, ultralong cycle life (1246 mAh g–1 after 900 cycles at 1000 mA g–1), and improved rate performance. This work may provide guidelines for preparing cobalt oxide-based anodes for LIBs.

1. Introduction

Rechargeable lithium-ion batteries (LIBs) have been considered as one of the most important energy storage devices because of their environmental benignity and high energy density.1 Recently, considerable attention has been devoted to looking for high-capacity alternatives to commercial layered graphite anodes with a low theoretical capacity of 372 mAh g–1 due to the ever-growing demand for highly efficient LIBs to power various electrical devices.2,3 Transition-metal oxides, which are based on a special conversion reaction mechanism, are promising alternative anode materials.1 Among a wide range of transition metal oxides, cobalt oxides, including CoO, Co2O3, and Co3O4, have attracted much attention due to their rich abundance, low cost, and high reversible capacities.4,5

However, the commercial utilization of metal oxides in LIBs is limited by their inferior cycling stability and poor rate performance because of the large volume change and low electrical conductivity during lithiation/delithiation processes.6 Designing composite materials composed of transition-metal oxides and carbonaceous materials, such as heteroatom-doped carbon, graphene, or carbon nanotubes, has been considered as an available method to surmount the above problems.7,8 It is suggested that carbonaceous materials can both enhance the electrical conductivity of composites and buffer the volume expansion of transition-metal oxides during lithiation/delithiation processes.9 Moreover, it is noteworthy that the particle size of metal oxides has an extremely significant effect on the electrochemical performance of composite anodes.10 The small particle size with high dispersion can endow the composite anodes with good electrochemical performance.11

Due to their controllable structures, tunable function, and large specific surface areas, metal–organic frameworks (MOFs), especially zeolitic imidazolate frameworks (ZIFs), have been widely used as precursors to synthesize carbonaceous structures.12,13 For example, Wang et al. fabricated ultrathin carbon nanosheets by calcining ZIF-8 in an argon atmosphere.14 The MOF-derived carbonaceous materials have flourished as advanced electrode materials in the energy storage field.15−17

Herein, a novel zero-dimensional/three-dimensional structure is prepared by combining CoOx QDs and ZIF-67-derived carbon (denoted as NP CoOx QDs/C). To date, construction of such NP CoOx QDs/C composite materials has not yet been reported. The designed anodes possess the following advantages. First, the carbon prepared by carbonization of ZIF-67 can effectively improve the electrical conductivity of composite anodes. Second, CoOx QDs growing in situ along with ZIF-67-derived carbon can advantageously reduce the diffusion distance of lithium ions and increase the lithium storage capacity. Third, the three-dimensional nanoporous structure can effectively maintain the structural integrity of anodes and prevent the aggregation of CoOx QDs during lithiation/delithiation processes. Consequently, the NP CoOx QDs/C composite anodes exhibit a high specific capacity of 1873 mAh g–1 after 200 cycles at 200 mA g–1 and enhanced rate performance. Besides, the anodes also present a long cycling life. The specific capacity can still remain at 1246 mAh g–1 at 1000 mA g–1 even after 900 cycles.

2. Results and Discussion

The synthetic process of NP CoOx QDs/C composites is illustrated in Figure S1. First, a typical strategy is used to fabricate ZIF-67 precursors. Subsequent carbonization of ZIF-67 is applied to prepare the ZIF-67-derived carbon. Benefiting from its unique three-dimensional structure, we in situ synthesize the CoOx QDs on ZIF-67-derived carbon by a precipitation reaction. The experimental details are presented in the Experimental Section. The structure and morphology of NP CoOx QDs/C composites have been investigated, as exhibited in Figure 1. Figure 1a displays the morphology of as-prepared carbon derived from ZIF-67. It is clear that the ZIF-67-derived carbon well inherits the original polyhedral architecture (shown in Figure S2), despite a slight distortion owing to the removal of organic linkers during the carbonization process.18 Furthermore, a transmission electron microscopy (TEM) image of ZIF-67-derived carbon is displayed in Figure S3a. The porous structure is conducive to inhibiting the growth and aggregation of CoOx QDs, as the precursor solution can be well absorbed into the porous carbon.19 The surface of ZIF-67-derived carbon becomes rough after compositing with CoOx QDs, as shown in Figure 1b. Moreover, the scanning electron microscopy (SEM) image at high magnification of NP CoOx QDs/C displayed in Figure S3b clearly indicates that CoOx QDs with a few nanometers are successfully loaded on carbon. The energy-dispersive X-ray spectrometry (EDS) spectrum of NP CoOx QDs/C composites indicates the presence of the expected elements (C, Co, and O), as illustrated in Figure S3c. The TEM image (Figure 1c) of NP CoOx QDs/C shows the composites present in the porous structure. The selected area electron diffraction (SAED) pattern exhibits rings of CoO, Co3O4, and carbon phases (Figure 1d), which is in accordance with the XRD results (Figure 2a). High-resolution transmission electron microscopy (HRTEM) images reveal that CoOx QDs are uniformly distributed on ZIF-67-derived carbon (Figure 1e,f). Lattice fringes with the interplanar distance of 0.213 nm can be assigned to the (200) plane of CoO, while the interplanar spacing of 0.243 nm belongs to the (311) plane of Co3O4. The lattice fringes with a distance of 0.341 nm are related to the (002) plane of graphitic carbon, and a distance of 0.204 nm corresponds to the (111) plane of Co. The ZIF-67-derived carbon with uniformly dispersed Co nanoparticles can efficiently enhance the electronic conductivity of NP CoOx QDs/C.20 In addition, EDS mappings show that C, Co, and O elements distribute homogeneously in NP CoOx QDs/C, which further demonstrates the uniform dispersion of CoOx QDs on ZIF-67-derived carbon, as portrayed in Figure 1g–j.

Figure 1.

Figure 1

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SEM images of (a) ZIF-67-derived carbon and (b) NP CoOx QDs/C, (c) TEM image of NP CoOx QDs/C, (d) SAED of NP CoOx QDs/C, (e, f) HRTEM images of NP CoOx QDs/C, and (g–j) EDS mapping image of NP CoOx QDs/C.

Figure 2.

Figure 2

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(a) XRD patterns of NP CoOx QDs/C and ZIF-67-derived carbon, (b) Raman spectrum of NP CoOx QDs/C, (c) TGA curve of NP CoOx QDs/C, and (d) nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curve of NP CoOx QDs/C.

The X-ray diffraction (XRD) patterns of NP CoOx QDs/C and ZIF-67-derived carbon are shown in Figure 2a. The XRD pattern of ZIF-67-derived carbon presents a combination of carbon with the (002) peak and Co with (111), (200), and (110) peaks, in agreement with previous literature.21 The XRD pattern of NP CoOx QDs/C confirms the coexisting crystallographic phases of ZIF-67-derived carbon with Co nanoparticles and CoOx QDs. The diffraction peaks at 18.9, 31.2, 55.6, 59.3, and 65.2° can be ascribed to the (111), (220), (422), (511), and (440) planes of Co3O4 (PDF #09-0418), respectively. Besides, the diffraction peaks centered at 36.4, 42.4, 61.5, 73.6, and 77.5° belong to the (111), (200), (220), (311), and (222) planes of CoO (PDF #09-0402). Raman spectra are recorded to further obtain the structural information of products. As illustrated in Figure 2b, the Raman spectra of NP CoOx QDs/C reveal two characteristic peaks at 1270 and 1548 cm–1, corresponding to the D and G bands of carbon, respectively.22,23 The G band at 1548 cm–1 is associated with graphitic carbon. The D band at 1270 cm–1 is ascribed to defects owing to the nanoporous structure of the composite materials. ID/IG = 2.18 indicates the highly defective or porous nature of NP CoOx QDs/C.24 The numerous defects and vacancies induced by the high-temperature carbonization can offer abundant active sites for facilitating the migration speed of lithium ions across the carbon, thus improving the electrochemical performance of the composite anodes. Thermal gravimetric analysis (TGA) is performed in an air atmosphere to determine the components of the fabricated composite materials. When the temperature increases to 800 °C, the mass of NP CoOx QDs/C composites is reduced by 35.78% (Figure 2c). The amount of reduction is the carbon content in NP CoOx QDs/C composites. The porosities of NP CoOx QDs/C composites are analyzed via the nitrogen adsorption and desorption isotherms (Figure 2d). As can be seen in Figure 2d, the synthesized composites show a typical type IV adsorption–desorption isotherm with an H4-type hysteresis loop, indicating the existence of huge quantities of mesopores in NP CoOx QDs/C.25 Besides, a rapid increase of the adsorbed volume in the relative pressure range of 0–0.1 can be observed, which shows that there are also some micropores in composites.26 The Brunauer–Emmett–Teller (BET) specific surface area of NP CoOx QDs/C is 110.53 m2 g–1. The large specific surface area and high porosity may offer adequate electrolyte/electrode contact to facilitate the diffusion of the electrolyte, which is conducive to enhancing the electrochemical performance. Furthermore, the pore size distribution of NP CoOx QDs/C is analyzed according to Barrett–Joyner–Halenda (BJH) theory.27Figure 2d shows the pore size distribution curve (inset) of NP CoOx QDs/C composites. Micropores and mesopores simultaneously exist in the composites, and the pore size of the composites mainly distributes in the mesoporous diameter region (9–20 nm). These results reveal that we have successfully fabricated the NP CoOx QDs/C composites.

X-ray photoelectron spectroscopy (XPS) is further conducted to analyze the elemental composition and chemical state of NP CoOx QDs/C composites. Figure 3a illustrates that C, Co, and O elements can be clearly observed in the XPS survey spectrum. Figure 3b exhibits the high-resolution C 1s XPS spectrum of NP CoOx QDs/C, in which the peak located at the binding energy of 284.60 eV belongs to the sp2- and sp3-hybridized C–C bond and two peaks with binding energies of 285.97 and 288.41 eV can be ascribed to the C–O and C=O bonds, respectively.28,29 In the high-resolution Co 2p spectrum, as illustrated in Figure 3c, the peak at 795.86 eV corresponds to the characteristic peak of Co 2p1/2 in cobalt oxide species, such as CoO and Co3O4, and two peaks at 779.50 and 780.88 eV can be attributed to the characteristic peaks of Co 2p3/2 in CoO and Co3O4, respectively.30 Other peaks are observed at 783.05 eV belonging to the metallic Co and 802.73 eV corresponding to the Co–O bond due to the partial surface oxidation of the composite materials. The high-resolution O 1s spectrum is deconvoluted into three peaks: the adsorbed oxygen at 529.66 eV, lattice oxygen at 531.44 eV, and defective oxygen at 532.95 eV, as shown in Figure 3d.

Figure 3.

Figure 3

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(a) XPS survey spectrum of NP CoOx QDs/C and (b–d) high-resolution XPS spectrum of C, Co, and O, respectively.

The electrochemical performance of NP CoOx QDs/C anodes for LIBs is investigated at ambient temperature. The electrochemical tests are performed in 2032 coin-type half-cells with lithium metal foil as the reference and counter electrodes. Figure 4a shows the cyclic voltammetry (CV) curves for the initial five cycles of NP CoOx QDs/C anodes at a scan rate of 0.1 mV s–1 in a potential range of 0.01–3 V versus Li/Li+. In the first cathodic scan, the peak at 0.83 V due to the formation of a solid electrolyte interface (SEI) film accompanied by the decomposition of the electrolyte disappears in the following cycles, implying that the SEI film formation occurs mainly during the first cycle, and it is the main cause of the initial irreversible capacity loss.20,31,32 The sharp cathodic peak at 0.93 V is associated with the conversion of CoOx with Li: CoO + 2Li+ + 2e– → Co + Li2O; Co3O4 + 8Li+ + 8e– → 3Co + 4Li2O.3,5 In the next four scans, the sharp cathodic peak splits into two broad peaks at around 0.94 and 1.25 V, which can be attributed to the consecutive electrochemical reactions during the lithiation process. In the anodic scans, two anodic peaks located at 1.30 and 2.24 V can be assigned to the gradual oxidation reaction of Co to Co2+ and Co2+ to Co3+. Apart from the first cycle, the CV curves are well overlapped with each other, which indicates the good cycling stability and favorable reversibility of NP CoOx QDs/C anodes for LIBs. Galvanostatic charge/discharge curves of the 1st, 2nd, 5th, 10th, 50th, and 100th cycle of NP CoOx QDs/C anodes at a current density of 200 mA g–1 in a voltage range between 0.01 and 3 V are exhibited in Figure 4b. It reveals that the first discharge capacity and charge capacity are 2041 and 1253 mAh g–1, respectively, corresponding to an initial Coulombic efficiency of 61.39%. The lower initial Coulombic efficiency is mainly attributed to the side reaction between NP CoOx QDs/C anodes and lithium ions and irreversible formation of the SEI film on the anode surface.26,33 All of the charge–discharge curves are highly consistent with the CV profiles in Figure 4a. Moreover, NP CoOx QDs/C anodes show a decrease in the initial five cycles and an increase in discharge capacity from 1080 mAh g–1 for the 5th cycle to 1489 mAh g–1 for the 100th cycle, which can be mainly ascribed to the reversible growth of the polymeric film during cycling.34Figure 4c displays the cyclic performances of NP CoOx QDs/C anodes and commercial Co3O4 and CoO anodes at a current density of 200 mA g–1 in a voltage range of 0.01–3 V. NP CoOx QDs/C anodes deliver a reversible specific capacity of 1873 mAh g–1 after 200 cycles. Furthermore, the Coulombic efficiency of NP CoOx QDs/C approaches 100% after the primary cycles, indicative of its good cycling stability. In contrast to the NP CoOx QDs/C anodes, both commercial Co3O4 and CoO anodes exhibit rapid capacity fading and inferior cycling stability. The improved cycling performance of NP CoOx QDs/C anodes highlights the structural advantages of nanosized CoOx QDs on ZIF-67-derived carbon. Except for the high capacity and stable cycling performance, NP CoOx QDs/C anodes also exhibit improved rate performance, as illustrated in Figure 4d. The discharge capacities are 1274, 1184, 1040, and 915 mAh g–1 at current densities of 100, 200, 500, and 1000 mA g–1, respectively. When the current density returns to 100 mA g–1, a discharge capacity as high as 1205 mAh g–1 is obtained after 24 cycles, suggesting that lithiation/delithiation processes of NP CoOx QDs/C anodes are reversible. The subsequent cycling of NP CoOx QDs/C anodes is still stable, and the discharge capacity can be stabilized at about 1451 mAh g–1 for more than 100 cycles (inset of Figure 4d). To evaluate the structural stability, the long-term cyclic performance of NP CoOx QDs/C anodes for LIBs at a higher current density of 1000 mA g–1 is further explored (Figure 4e). Noticeably, the NP CoOx QDs/C anodes maintain a high specific capacity of 1246 mAh g–1 after 900 cycles at 1000 mA g–1, which is 17.3 times the theoretical capacity of commercial graphite. This result further demonstrates the stable cycle performance of NP CoOx QDs/C anodes. The capacity contribution rate of ZIF-67-derived carbon to NP CoOx QDs/C anodes is almost negligible as reported in the literature.35 The ultrahigh specific capacity of NP CoOx QDs/C is mainly attributed to CoOx QDs. A capacity lifting phenomenon can be observed upon cycling for NP CoOx QDs/C anodes (Figure 4c,e), which is a typical phenomenon for oxide and chalcogenide anodes.36 Capacity lifting can be ascribed to the reversible growth of the polymeric film on the active surface of anode materials, the formation of the Li-containing SEI film, and the defect-induced lithium storage.33,34,36,37 In our case, the capacity lifting phenomenon of NP CoOx QDs/C upon cycling might be attributed to the reversible growth of the polymer film on the surface of anode materials.38 The lithium storage performance of NP CoOx QDs/C anodes is compared with that of reported cobalt oxide-based anode materials, as shown in Table 1. The lithium storage performance of NP CoOx QDs/C anodes is nearly superior to that of these anode materials.

Figure 4.

Figure 4

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Electrochemical characterization of NP CoOx QDs/C anodes. (a) CV curves for the initial five cycles of NP CoOx QDs/C at 0.1 mV s–1 within 0.01–3 V versus Li/Li+. (b) Galvanostatic charge/discharge curves of NP CoOx QDs/C anodes at 200 mA g–1. (c) Cyclic performances of NP CoOx QDs/C anodes and commercial CoO and Co3O4 anodes at 200 mA g–1 from 0.01 to 3 V. (d) Rate performance of NP CoOx QDs/C anodes from 100 to 1000 mA g–1. (e) Long-term cyclic performance of NP CoOx QDs/C anodes at 1000 mA g–1 between 0.01 and 3 V.

Table 1. Comparison of the Lithium Storage Performance of Some Cobalt Oxide-Based Materials.

references materials capacity (mAh g–1) cycles current densities (mA g–1)
(3) CoO@C 1003 200 200
(4) Mn/Ni Co-doped CoO@C 1126 1000 1000
(5) Co3O4 nanoparticles 880 50 50
(21) CoO-NCNTs 583 2000 500
(28) CoO@N,S-codoped carbon 809 500 1000
(34) C-doped Co3O4 HNFs 1121 100 200
(39) Co3O4/N-C 423 100 100
(40) Ti-doped CoO@C 1108 150 200
(41) CoO/graphene 640 150 100
(42) oxygen-defective Co3O4 896 200 250
(43) CoC2O4@CoO/Co 802 200 200
(44) CNF/CoO 530 100 200
(45) Co3O4 nanotubes 1081 50 100
(46) Co3O4@Co@GC 749 60 200
this work NP CoOx QDs/C 1873 200 200
this work NP CoOx QDs/C 1246 900 1000
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To understand the lithium storage behavior of NP CoOx QDs/C anodes, a kinetics analysis is performed on CV measurements. Figure 5a shows the CV curves at different scan rates from 0.1 to 0.5 mV s–1. The CV shape nearly remains unchanged even as the scan rate increases, implying the good kinetics reversibility. A diffusion-controlled insertion process and a surface-controlled capacitance process usually serve as two separate mechanisms to describe the charge-storage kinetics.47−49 The contribution of capacitance during the lithium storage process can be qualitatively evaluated using the power-law formula

2. 1

where i is the peak current, ν is the scan rate, and a and b are positive variables.50 The b value can be obtained on the basis of the slope of the log ν −log i plot. Typically, the b value of 0.5 means a diffusion-controlled insertion process, whereas that of 1.0 represents a surface-controlled capacitance process. The b value between 0.5 and 1.0 suggests a mixed mechanism during the charge-storage process. From Figure 5b, the b values of two peak positions (peaks 1 and 2) are 0.773 and 0.857, respectively, which means the co-existence of both diffusion-controlled insertion process and surface-controlled capacitance process. Furthermore, the contributions from the two mechanisms during the lithium storage process at a fixed potential can be analyzed by the following equation

2. 2

where k1 and k2 are constants and k1ν and k2ν0.5 are related to the surface-controlled capacitance process and the diffusion-controlled insertion process, respectively.51−53 As illustrated in Figure 5c,d, 60.41 and 80.99% of the total charge are contributed by the surface-controlled capacitance process at scan rates of 0.1 and 0.5 mV s–1, respectively. Figure 5e presents the contribution percentages at scan rates of 0.1, 0.2, 0.3, 0.4 and 0.5 mV s–1. Obviously, as the scan rates increase, the surface-controlled capacitive contribution is gradually improved, with values of 60.41, 67.11, 72.02, 77.01, and 80.99% at scan rates of 0.1, 0.2, 0.3, 0.4 and 0.5 mV s–1, respectively. The results indicate that the surface-controlled capacitance process dominates the whole capacity, which can greatly boost the lithium storage capacity and effectively enhance the rate performance of NP CoOx QDs/C anodes.

Figure 5.

Figure 5

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Kinetics analysis of the lithium storage behavior for NP CoOx QDs/C anodes. (a) CV curves at different scan rates. (b) Plots of log (peak current) versus log (scan rate) at different redox states. (c, d) Capacitive contribution (purple region) and diffusion at scan rates of 0.1 and 0.5 mV s–1, respectively. (e) Normalized contribution ratio of capacitance and diffusion at different scan rates. (f) Nyquist plots for NP CoOx QDs/C anodes obtained before and after 60 cycles at a current density of 200 mA g–1.

In addition, electrochemical impedance spectroscopy (EIS) measurements are conducted to gain insight into the robustness of the electrochemical performance of NP CoOx QDs/C anodes. Figure 5f shows the Nyquist plots of NP CoOx QDs/C anodes before and after 60 cycles in a full charge state. Both spectra consist of semicircles in the high- to medium-frequency region and inclined straight lines in the low-frequency region. According to previous literature, the semicircles are related to the resistance of the SEI film (RSEI) and the charge-transfer resistance (Rct), and the straight lines can be assigned to the lithium-ion diffusion in electrodes, called the Warburg impedance (Zw).54,55 As illustrated in Figure 5f, both the fresh and after-cycled NP CoOx QDs/C anodes show a relatively low Rct, suggesting the high electrical conductivity of the NP CoOx QDs/C anodes. Therefore, NP CoOx QDs/C anodes display dramatically improved electrochemical performance, which can be ascribed to the rational structural design of NP CoOx QDs/C. The three-dimensional nanoporous structure endows the NP CoOx QDs/C composite anodes with reduced ion diffusion paths, high structural stability, and effective electrolyte penetration.56,57 Moreover, the Rct of the NP CoOx QDs/C anodes (205 Ω) after 60 cycles is smaller than that of fresh NP CoOx QDs/C anodes (316 Ω), which might be attributed to the structure evolution of NP CoOx QDs/C upon cycling. Furthermore, the SEM image of NP CoOx QDs/C anodes after 200 cycles shown in Figure S4a reveals that the NP CoOx QDs/C anodes well maintain the structural integrity, which is vital for improving the electrochemical performance of volume-sensitive cells. Moreover, the SEM image of NP CoOx QDs/C reveals that the porous morphology of NP CoOx QDs/C is well retained (Figure S4b).

The electrochemical performance of NP CoOx QDs/C anodes can be attributed to the rational structural design, as illustrated in Figure 6. First, carbonized ZIF-67 with uniformly dispersed Co nanoparticles can greatly improve the electronic conductivity of NP CoOx QDs/C, enhancing the rate performance. Second, CoOx QDs can shorten the electronic and ionic transport distance, further boosting the rate performance. Third, NP CoOx QDs/C composites with well-dispersed CoOx QDs immobilized firmly on carbon are conducive to inhibiting the aggregation and volume change of CoOx upon cycling, which can benefit the cycling performance of composite anodes. Finally, the nanoporous structure can shorten the ion diffusion paths and maintain the structural integrity during lithiation/delithiation processes, thereby promoting the electrochemical performance.

Figure 6.

Figure 6

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Schematic illustration of lithiation for NP CoOx QDs/C anodes.

3. Conclusions

In summary, we have successfully fabricated three-dimensional nanoporous composites of CoOx quantum dots and ZIF-67-derived carbon by a facile in situ precipitation method. The unique three-dimensional construction of NP CoOx QDs/C composite materials endows the anodes with shortened lithium-ion diffusion length, improved electrochemical conductivity, and good structural stability. Therefore, the NP CoOx QDs/C anodes exhibit a high reversible capacity of 1873 mAh g–1 after 200 cycles at 200 mA g–1 and enhanced rate performance. Furthermore, the NP CoOx QDs/C anodes present a long cycle life. The specific capacity can still remain at 1246 mAh g–1 at 1000 mA g–1 even after 900 cycles.

4. Experimental Section

4.1. Fabrication of ZIF-67-Derived Carbon

In a typical experiment, 0.520 g of cobalt chloride hexahydrate (CoCl2·6H2O) was dissolved in 40 mL of methanol to form a pink colored solution. Then, 2.630 g of 2-methylimidazole was added into another 40 mL of methanol to prepare a clear solution. Next, these two solutions were mixed and stirred in air for 0.5 h. After aging for 24 h, the obtained purple precipitates (ZIF-67) were collected by washing with methanol three times before vacuum drying at 60 °C for 8 h. The product was annealed at 630 °C (heating rate 3 °C min–1) for 3 h under an argon atmosphere. After cooling down naturally, the product was purified by hydrochloric acid (HCl), and the ZIF-67-derived carbon was fabricated.

4.2. In Situ Growth of CoOx QDs on ZIF-67-Derived Carbon

Typically, 320 mg of cobaltous acetate (Co(CH3COO)2) was dispersed in 14 mL of benzyl alcohol under stirring for 2 h. Then, 0.072 mg of ZIF-67-derived carbon was immersed in the above solution. After ultrasonic dispersion for 0.5 h, 14 mL of ammonium hydroxide (NH3·H2O, 25 wt %) was added into the solution dropwise under vigorous stirring and maintained for 0.5 h. The mixture was placed in an oil bath at 165 °C for 2 h. After cooling to room temperature, the NP CoOx QDs/C was prepared.

4.3. Materials Characterization

XRD (Rigaku Dmax-rc), Raman (LabRAM HR800 spectrometer), and XPS (ESCALAB 250) were used to investigate the crystal phase and chemical state of samples. Morphology and composition analyses were conducted with SEM (Zeiss SUPRA 55)-attached EDS, TEM (FEI Tecnai G2 F20), and HRTEM (JEOL JEM-2010). TGA was performed in air from 35 to 900 °C (heating rate 10 °C min–1). The specific surface area was measured by an ASAP 2020 HD88 instrument.

4.4. Electrochemical Tests

Coin-type (CR 2032) cells were assembled to investigate the electrochemical performance with Li foil as the counter electrode, NP CoOx QDs/C composite as the working electrode, and Celgard 2400 as the separator. Active materials, acetylene black, and poly(vinylidene fluoride) were dispersed in N-methyl-2-pyrrolidone at the ratio of 7:2:1 (wt %). The slurry was loaded on a Cu current collector and then dried at 60 °C overnight. The mass loading of active materials was about 1.1 mg cm–2. The electrolyte was 1.0 M LiPF6 solution in dimethyl carbonate, ethylene carbonate, and diethyl carbonate (1:1:1 vol %). Galvanostatic charge/discharge tests were performed on a lithium battery cycler (Neware CT-4008, Shenzhen China) between 0.01 and 3 V. EIS and CV were carried out on an electrochemical workstation (CHI 660E, Shanghai China).

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 51871135).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c02037.

  • Schematic illustration of the preparation of NP CoOx QDs/C; SEM and TEM images of ZIF-67; TEM image of ZIF-67-derived carbon; SEM image of NP CoOx QDs/C; EDS pattern of NP CoOx QDs/C; SEM images of NP CoOx QDs/C anodes after 200 cycles and NP CoOx QDs/C after 200 cycles (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c02037_si_001.pdf (238.2KB, pdf)

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