Hydrothermal Synthesis of Hierarchical Cage-like Co₉S₈ Microspheres Composed of Nanosheets as High-Capacity Anode Materials

Abstract

Co₉S₈ is considered to be one of the most promising anode materials because of its high theoretical capacity. In this work, hierarchical cage-like Co₉S₈ microspheres composed of well-crystallized nanosheets are successfully synthesized at 180 °C by a hydrothermal method using KOH and disodium ethylenediamine tetraacetate (Na₂EDTA) as a mineralizer and a complexing agent, respectively. X-ray diffraction and scanning electron microscopy measurements show that KOH is beneficial in promoting the crystallization and development of Co₉S₈, avoiding the formation of impurities, while Na₂EDTA is conducive to the generation of cage-like microspheres with the micro/nano architecture and better crystallization. The unique hierarchical cage-like micro/nano architecture can effectively relieve the volume change in the cycling process, and the well-crystallized Co₉S₈ nanosheets in the cage-like microspheres can offer much more active sites for Li⁺ accommodation, and thus the hierarchical cage-like Co₉S₈ microspheres composed of well-crystallized nanosheets show superior cycling stability and rate capability, e.g., a high capacity of 303.5 mAh g⁻¹ after 1000 cycles at a high rate of 1.0 A g⁻¹. This work provides a new approach for improving the electrochemical performance of LIBs by constructing a hierarchical anode material.

1. Introduction

Lithium-ion batteries (LIBs) are currently extensively utilized in portable electronic equipment, power grids, and electric vehicles [1,2,3,4,5]. However, with the rapid development of modern technology and the improvement of people’s living standards, graphite anode materials fall short of meeting the demands of future LIBs because of their low specific capacity (372 mAh g⁻¹), limited rate capability, and easy growth of lithium dendrite [6]. As alternative anode materials to graphite, transition-metal sulfides (TMSs) have obvious advantages such as a relatively high theoretical capacity, natural abundance, affordability, good safety, and easy preparation [7,8,9,10]; thus, they have attracted much attention. Among them, cobalt sulfide (Co₉S₈) is considered as one of the most promising anode materials due to its high theoretical capacity (545 mAh g⁻¹) and high electronic conductivity [11]. Nevertheless, micro-sized Co₉S₈ particles usually exhibit insufficient electrochemical performance because of the low specific surface area and the resulting sluggish electrochemical reaction kinetics [12].

To tackle this issue, a lot of nanosized materials are exploited as electrodes in LIBs because of the following advantages: (1) the diffusion time of Li⁺ can be shortened by decreasing the dimensions, which facilitates reaction kinetics for high charge–discharge rates; (2) their large surface area can promote interfacial Faradaic reactions within the batteries and the movement of Li⁺ across the electrode/electrolyte interface, resulting in improved capacity; and (3) their nanostructure can alleviate the structural change during lithiation/delithiation, protecting against pulverization of the electrode material [13]. But nanosized materials also have their own shortcomings, such as difficulty in controlling their dimensions, a low volumetric energy density, more unwanted side reactions at the electrode/electrolyte interface, and aggregation between the nanoparticles [14]. The same problems also occur in the case of nanosized Co₉S₈ anode materials. Thus, it is of significance to exploit a novel structure combining the advances of nanostructures and microstructures to enhance the electrochemical performance of Co₉S₈ anode materials.

It has been proved that active microparticles with a hierarchical micro/nano architecture usually possess a better electrochemical performance because the hierarchical structure can not only provide nanosized domains for fast ion diffusion but also maintain effective particle packing at the micron-scale to restrict the self-aggregation and cracking during the repeated charge–discharge process [15,16]. For instance, a hierarchical Li₄Ti₅O₁₂ microsphere composed of nanosheets shows a high tap density of 1.32 g cm⁻³ and an exceptionally high rate capability of 155 mAh g⁻¹ even at 50 C. Moreover, it shows 99.5% capacity retention after 2000 cycles at 50 C and 95.4% capacity retention after 3000 cycles at 30 C, displaying the outstanding cycling stability [17].

Various strategies have been exploited for the fabrication of 3D microarchitectures. Among these approaches, the hydrothermal synthesis approach has shown high effectiveness in achieving materials with controlled morphologies [18]. Notably, a few small chelating agents such as disodium ethylenediamine tetraacetate (Na₂EDTA) and citric acid (CA) with strong self-assembly capabilities in aqueous solution play a crucial role in determining the final product’s morphology [19,20]. Nevertheless, these chelating agents have not yet been utilized to prepare Co₉S₈ particles with a hierarchical micro/nano architecture.

In this study, a significant attempt was made to synthesize cage-like Co₉S₈ anode materials with a hierarchical micro/nano architecture by a hydrothermal process, in which KOH was used as a mineralizer and disodium ethylenediamine tetraacetate (Na₂EDTA) was used as a complexing agent to control the crystallization and the growth of Co₉S₈. In a good agreement with our anticipation, the cage-like Co₉S₈ microspheres with the micro/nano architectures as anode materials in LIBs showed excellent electrochemical performance.

2. Experimental Section

2.1. Material Synthesis

For the material’s synthesis, 0.0025 mol cobalt chloride hexahydrate (CoCl₂·6H₂O) and 0.005 mol thiourea (CS(NH₂)₂) were weighed and dissolved in 50 mL of distilled water. Subsequently, 0.025 mol potassium hydroxide (KOH) was introduced into the mixture, and the solution was stirred magnetically for 15 min. The resulting solution was transferred into a 100 mL teflon-lined autoclave, sealed, and heated to 180 °C for 24 h. After the hydrothermal process, the product was filtered and washed several times with distilled water and absolute ethanol, followed by drying at 70 °C for 6 h to obtain the final product, which was named No-EDTA. In addition, a 40-EDTA sample was prepared by a similar process, in which 0.0025 mol CoCl₂·6H₂O was dissolved in 40 mL of distilled water, and then 0.0025 mol Na₂EDTA was added, heated, and stirred until a clear solution was obtained. Subsequently, CS(NH₂)₂ in an amount twice the molar quantity of CoCl₂·6H₂O, 0.025 mol KOH, and 10 mL ethanol was introduced into the former solution. The other procedure was the same as that of No-EDTA. Similarly, a 50-EDTA sample was prepared by repeating the synthesis procedure for the 40-EDTA under the same conditions using 50 mL water as the solvent in the autoclave without ethanol.

2.2. Material Characterization

The XRD patterns of the synthesized samples were collected using an X-ray diffractometer (XRD, D8 focus, Bruker Corporation, Billerica, MA, USA) with Cu Kα radiation, scanning over a 2θ range from 10° to 80°. Their morphology and microstructure were observed using a scanning electron microscope (SEM, Apreo 2C, Thermo Fisher Scientific, Waltham, MA, USA).

2.3. Electrochemical Performance

The electrochemical tests were conducted by assembling the CR2032 coin-type cells in an argon-filled glovebox. The active materials (Co₉S₈), along with acetylene black as a conductive agent and polyvinylidene fluoride (PVDF) as a binder, were mixed in a weight ratio of 70:20:10. This mixture was ground in N-methyl-pyrrolidone (NMP) to form a slurry, which was coated on a copper foil and dried at 100 °C in a vacuum overnight to prepare the working electrodes. The loading of the active material on the electrode was approximately 1–2 mg cm⁻². Here, polypropylene films were used as separators. The electrolyte consisted of a 1.0 M LiPF₆ solution with a solvent mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) in a 1:1:1 volume ratio. Galvanostatic discharge–charge tests were performed using a CT2001A battery-testing system (LAND Electronic Co., Wuhan, China) to evaluate the electrochemical performance. Cyclic voltammograms (CVs) were measured by a CHI-660E electrochemical workstation.

3. Results and Discussion

The SEM images of the No-EDTA, 40-EDTA, and 50-EDTA samples are given in Figure 1, which clearly displays the evolution process of the Co₉S₈ microstructure with the processing parameters. As shown in Figure 1a,b, the direct hydrothermal process without using any complexing agent resulted in an irregular and aggregated morphology and wide size distribution of the Co₉S₈ crystallites. Nevertheless, when Na₂EDTA was used as the complexing agent and a small amount of ethanol was introduced in the hydrothermal process, a big change in the Co₉S₈ crystallites’ morphology could be seen, i.e., a nanosheet-assembled cage-like microstructure was preliminarily formed, though they were still in a poor crystalline state (Figure 1c,d). When only distilled water was used as a solvent, the 50-EDTA sample displayed typical cage-like microspheres that were ca. 5 μm in diameter, and this hierarchical microstructure was composed of Co₉S₈ nanosheets (see Figure 1e,f). The XRD results also proved the good crystallization of the 50-EDTA sample.

Figure 2 shows the XRD patterns of the as-prepared samples under different hydrothermal conditions. It can be seen that cubic-phase Co₉S₈ (JCPDS Card, No. 86-2273) can be easily obtained at 180 °C by the direct hydrothermal reaction of CoCl₂·6H₂O, CS(NH₂)₂, and KOH in 50 mL distilled water, and no other impurity can be found (Figure 2a). Moreover, the presence of the mineralizer of KOH in the hydrothermal process promoted the crystallization and development of Co₉S₈ crystallites [21]. The XRD patterns of the 40-EDTA and 50-EDTA samples (see Figure 2b,c) show the formation of pure cubic-phase Co₉S₈ crystallites irrespective of adding ethanol or not, and their diffraction peak intensity increases with the decrease in ethanol in mixed solvent in the presence of Na₂EDTA. This indicates that the introduction of ethanol would inhibit their crystallization and growth, and that large Co₉S₈ crystallites can be obtained with only distilled water as the solvent. In addition, the comparison of XRD patterns between the No-EDTA and 50-EDTA samples (Figure 2a,c) also confirmed that Na₂EDTA was beneficial for the crystallization and growth of Co₉S₈ crystallites since their diffraction peak intensity obviously increased with the introduction of Na₂EDTA.

And the average crystalline size (D) was determined using the Scherrer equation [22]:

$$D={\displaystyle \frac{K·\lambda }{\beta ·\cos \theta }}$$

(1)

where λ is the X-ray wavelength, the K is 0.94, β is defined as the full-width at half maximum of the X-ray peak, and θ is the position of the peak. For the calculations, the (311) peak of the cubic-phase Co₉S₈ is used. As shown in

Table 1. Calculated crystalline size of 50-EDTA, 40-EDTA, and No-EDTA using the Scherrer equation.

Sample	Crystallite Size (nm)
50-EDTA	17.03
40-EDTA	15.41
No-EDTA	14.72

, the crystalline size of 40-EDTA is smaller than that of 50-EDTA, confirming that ethanol could inhibit the crystallization and growth. Moreover, the crystalline size of 50-EDTA is larger than that of No-EDTA, confirming that Na₂EDTA is beneficial for crystallization and growth.

Based on the abovementioned SEM and XRD results, the effects of Na₂EDTA and ethanol on the Co₉S₈ samples are as follows. Na₂EDTA was conducive to the crystallization and growth of Co₉S₈ crystallites since it can coordinate metal Co²⁺ ions to form stable Co-Na₂EDTA intermediate complexes [23]. Under the subsequent hydrothermal conditions, Co₉S₈ crystal nuclei would slowly form and gradually grow into uniform nanosheets, due to the change in the surficial free energy, the stabilization of certain crystallographic planes by Na₂EDTA, and the intrinsic anisotropic growth [20]. In addition, Na₂EDTA can induce the self-assembly of 2D nanosheets into 3D hierarchical cage-like microstructures [19]. Moreover, water has a higher surface tension (γ) of (72.7 mN/m) than ethanol (22.3 mN/m) [24], so the Co₉S₈ nanosheets became irregular (like granulates) to reduce the surface Gibbs free energy when only using water as the solvent. However, the 50-EDTA sample still maintained a hierarchical micro/nano architecture.

The variation in electrochemical performance with the Co₉S₈ crystalline morphology was also investigated in detail by constructing them as the anode materials. Cyclic voltammetry (CV) curves were obtained over a range of 0.01 to 3 V at a scan rate of 0.2 mV/s to clarify the electrochemical reaction mechanism. Figure 3a shows the CV profiles of the No-EDTA sample for the first three cycles. Different from the second and third cycle, a weak cathodic peak appears at 0.67 V in the initial discharge cycle, and it can be assigned to the irreversible formation of the solid electrolyte interface (SEI) between the electrode and the electrolyte, which generally leads to an irreversible capacity loss and a relatively low Coulombic efficiency (CE) in the initial cycle [25]. Another cathodic peak at 1.16 V corresponding to the Co₉S₈ is reduced to metallic Co and the insertion of Li⁺ into Co₉S₈ forms Li_xCo₉S₈. Two anodic peaks in the first charge cycle at approximately 2.05 V and 2.39 V are associated with the reversible oxidation reaction of metallic Co to Co₉S₈, which indicates that the charge procedure is a two-step process. The charge products are converted to intermediates at the voltage of 2.05 V, and then the intermediates are oxidized to Co₉S₈ and Li [26,27]. In the subsequent scans (the second and third cycle), both cathodic peaks shift toward the larger voltages owing to the structure rearrangement during the initial discharge reaction or decrease in the electrode polarization [28]. However, the voltage gaps between the cathodic peaks and the anodic peaks become smaller, indicating that the electrochemical polarization weakens and the electrochemical reversibility gets better.

The CV curves corresponding to the electrodes prepared by 40-EDTA and 50-EDTA are shown in Figure 3b,c, and they are a little different from that of No-EDTA. There is no obvious anodic peak (2.39 V) related to the formation and decomposition of intermediates in the first three cycles. Their cathodic peaks at about 1.04 V in the initial cycle are also attributed to the reduction of Co₉S₈ to Co and Li₂S. In addition, similar to that of the No-EDTA sample, there are two weak cathodic peaks, at 0.65 V in Figure 3b and 0.68 V in Figure 3c, respectively, which are also assigned to the irreversible formation of the solid electrolyte interface (SEI). In the second and third cycle, both anodic and cathodic peaks shift toward the right, and the voltage gaps between their cathodic peaks become smaller, while the overlapping degree of their anodic peaks in the first three cycles is higher than that of No-EDTA, demonstrating the enhanced electrochemical reversibility and smaller electrochemical polarization [27], especially for the 50-EDTA sample.

According to the CV results, the discharge–charge reaction mechanism of Co₉S₈ can be described as follows [26]:

In the discharge process,

Co₉S₈ + 16Li⁺ + 16e⁻ → 8Li₂S+ 9Co

(2)

In the charge process,

9Co + 8Li₂S → Co₉S₈ + 16Li

(3)

The galvanostatic discharge–charge profiles in the first, second, and third cycle are shown in Figure 3d,e, and f in the voltage range of 0.01–3 V at 0.1 A g⁻¹. The charge/discharge specific capacities of the No-EDTA, 40-EDTA, and 50-EDTA samples in the first cycle are 694.2/894.5, 626.5/754.9, and 763.9/932.5 mAh g⁻¹, respectively, and the corresponding CEs are 77.6%, 83.0%, and 81.9%, respectively. The relatively low CEs can be ascribed to the SEI formation and the irreversible decomposition of electrolytes [29], as confirmed by the CV measurements in Figure 3a–c. In the following cycles, their CEs gradually increased and steadily maintain close to 100%. The 50-EDTA sample has the highest initial charge/discharge capacities, which seems to be strongly due its unique cage-like hierarchical microstructure. In the second and third cycles, their charge and discharge curves are similar, implying that the transfer process of Li⁺ is of good reversibility for the subsequent cycles. Furthermore, there is an obvious discharge potential plateau at ca. 1.2 V and a weak one at ca. 0.65 V in the initial discharge curves, which correspond to the reaction of Co₉S₈ to metallic Co and the formation of the SEI layer; meanwhile, another potential plateau can be seen at ca. 1.95 V in the first charge curves. All the results are in good agreement with the above analysis of the CV profiles.

As is known, evaluating the cycling performance of Co₉S₈ samples is of significance. As displayed in Figure 4a, the experimental data were obtained over 0.01–3 V at a current density of 0.1 A g⁻¹. All the cyclic trends are similar, and the specific capacity decreases in the first 30 cycles due to the slow activation of the electrodes and the formation of the SEI in the reciprocating lithiation/delithiation processes. Subsequently, the specific capacity increases gradually with the increase in the cycling number. The activation of the electrode for Li⁺ diffusion leads to an increase in specific capacity [26]. Apparently, the specific capacity is closely related to the samples’ microstructure under the same chemical and phase composition. The irregular morphology and wide particle-size distribution of the No-EDTA sample resulted in rapid decaying in the beginning, with a much lower specific capacity than the other two samples after the 10th cycle. The cage-like microstructure composed of uniform Co₉S₈ nanosheets favors the improvement in the specific capacity and meanwhile could effectively suppress the pulverization due to the change in material volume [11]. Therefore, the 40-EDTA sample in a poor crystalline state exhibits better cycling performance. In contrast, the 50-EDTA sample exhibits the best cycle performance among the three samples because of the well-crystallized cage-like microspheres formed with the Co₉S₈ nanosheets, which not only offer more active sites for Li⁺ accommodation [28] but also avoid microstructure destruction during the cycling process [17].

Figure 4b depicts the rate performance of the Co₉S₈ samples with a current density ranging from 0.1 A to 2.0 A g⁻¹ for 10 cycles. The 50-EDTA sample shows a capacity of 673.7, 514.1, 393.2, 288, and 202.6 mAh g⁻¹ at a current density of 0.1, 0.3, 0.5, 1.0, and 2.0 A g⁻¹, respectively, which is much higher than that of the other two samples. When the current density returned back to 0.1 A g⁻¹, the specific capacity almost recovered to the initial value, which is ascribed to the well-crystallized cage-like hierarchical microstructure. It is noteworthy that the 40-EDTA sample also had a good rate performance due to its uniform nanosheet structure, which is helpful for facilitating Li⁺ diffusion and reducing the polarization [30], but its cage-like microspheres had just formed and were still in a relatively low crystallinity state, so the rate performance was worse than that of 50-EDTA. In contrast, the No-EDTA sample shows the poorest rate performance because of the absence of the hierarchical micro/nano architectures and the poor crystallization of Co₉S₈.

Furthermore, the long-term cycling stability of the 50-EDTA electrode is also shown in Figure 5. The 50-EDTA electrode maintains a high capacity of 303.5 mAh g⁻¹ after 1000 cycles even at a high rate of 1.0 A g⁻¹, showing promising potential as an anode material in high-capacity LIBs. As shown in

Table 2. Comparison of cycling performance of Co₉S₈ materials reported previously.

Materials	Current Density (mA g−1)	Cycle Number	Remaining Capacity (mAh g−1)	Ref.
C-Co9S8	1090	300	277	[27]
Co9S8 nanoparticle	1000	100	100	[31]
Co9S8 microsphere	50	50	391	[32]
Co9S8 electrode	1000	2000	132	[33]
Co9S8	2000	250	245	[34]
50-EDTA	1000	1000	303.5	This work

, the electrochemical performance of the 50-EDTA electrode is also superior to other Co₉S₈ materials reported previously [27,31,32,33,34].

4. Conclusions

Cage-like hierarchical Co₉S₈ microspheres composed of crystalline nanosheets were successfully synthesized via a facile hydrothermal method. The addition of KOH avoided the formation of impurities and promoted the crystallization and development of Co₉S₈ crystallites. Na₂EDTA was beneficial to the construction of cage-like microspheres and the crystallization and growth of Co₉S₈ nanosheets. On the contrary, introducing ethanol would inhibit the crystallization and the formation of cage-like microspheres. Because of the unique cage-like micro/nano architectures with high specific areas and the effective relief of the volume change in the cycling process, the 40-EDTA and 50-EDTA samples exhibited better electrochemical performance than the No-EDTA sample. In particular, the 50-EDTA electrode showed the highest specific capacity and rate capability because of the well-crystallized Co₉S₈ nanosheets with more active sites. This study provides a new route to improving the electrochemical performance of LIBs by constructing a cage-like hierarchical micro/nano architecture as an anode material.