Nanostructured C@CuS Core–Shell Framework with High Lithium-Ion Storage Performance

Abstract

In this study, we have synthesized a nanostructured core–shell framework of carbon-coated copper sulfide (C@CuS) through a one-step precipitation technique. The carbon sphere template facilitated the nucleation of CuS nanostructures. The synthesized nanocomposites have demonstrated remarkable lithium-ion storage capabilities when utilized as an anode in lithium-ion batteries. Notably, they exhibit an impressive rate capability of 314 mAh g⁻¹ at a high current density of 5000 mA g⁻¹, along with excellent long-term cycle stability, maintaining 463 mAh g⁻¹ at 1000 mA g⁻¹ after 800 cycles. This superior performance is due to the core–shell architecture of the composite, where the carbon core enhances the conductivity of CuS nanoparticles and mitigates volume expansion, thus preventing capacity loss. Our study not only elucidates the significance of carbon in the construction of nano-heterojunctions or composite electrodes but also presents a practical approach to significantly boost the electrochemical performance of CuS and other metal sulfides.

1. Introduction

The increasing demands for fast charging and discharging performance, high energy density, and power density in lithium-ion batteries (LIBs) have surpassed the capabilities of commercial batteries [1,2,3]. Therefore, there is an urgent need to develop new materials with improved electrochemical performance for the next generation of LIBs. The anode materials play a crucial role in determining the electrochemical performance of batteries. In recent years, various materials such as silicon, lithium metal, metal oxides, and metal sulfides have been proposed as potential replacements for graphite anodes in LIBs [4,5,6,7,8].

Transition metal sulfides, specifically M_xS_y (M = Mn, Fe, Ni, Co, Cu), exhibit a theoretical specific capacity that is twice as high as that of commercial graphite electrodes. Moreover, most metal sulfides have a voltage plateau above 1.0 V, which prevents the formation of lithium dendrites and ensures safety [9]. These characteristics make transition metal sulfides promising candidates for the anode materials in next-generation LIBs [10,11,12]. However, their poor cycle performance and rate performance limit their applications, which are due to volume expansion during charging and discharging and low conductivity [13,14,15]. To overcome these challenges, several effective strategies have been proposed, including nano crystallization [16], hollow nanostructure design [17], composite materials [18], and the fabrication of heterojunctions [19].

The combination of transition metal sulfides with carbon as hybrid materials has shown potential in improving cycle life and rate performance. By incorporating carbon, the volume expansion caused by lithium intercalation can be alleviated, thus the integrity of the structure is maintained [20,21,22]. For instance, Wang et al. synthesized a carbon nanotubes/CuS (CNTs/CuS) composite material with excellent cycle performance using a microwave-assisted hydrothermal method [23]. This composite exhibited a reversible capacity of 569 mA h g⁻¹ after 450 cycles at a current density of 400 mA g⁻¹. Li et al. prepared a CuS/graphene composite via a one-pot microwave-assisted method, demonstrating enhanced cycle stability and rate performance with a capacity of 348 mAh g⁻¹ at a current density of 2000 mA g⁻¹ after 1000 cycles [19]. Zhang et al. synthesized a CuS/reduced graphene oxide (rGO) composite through a stepwise method which exhibited a high initial discharge specific capacity of 882 mAh g⁻¹ along with a stable cycle performance and high coulombic efficiency [24]. The improved electrochemical performance of these composites is primarily attributed to the introduction of carbon materials, which not only alleviate volume expansion but also enhance conductivity.

However, using high-cost materials like graphene or carbon nanotubes to prepare CuS-based composites is not very economical. Additionally, the complex and costly preparation processes, involving multiple steps, hinder the industrialization and widespread application of these composites. Therefore, there is a need to develop a simple, inexpensive, and scalable synthesis method for CuS-based composites. In this study, we present a one-step precipitation method to fabricate C@CuS composites, incorporating a carbon sphere template for nanostructured CuS heterogeneous nucleation. Acetylene black (C45), a cheaper alternative to graphene or carbon nanotubes, was used as a carbon source in the experiment. Moreover, the prepared composites exhibit good electrochemical properties. During the synthesis process, CuS nanoparticles quickly nucleate and grow on the carbon skeleton assembled with carbon nanoparticles, ensuring the formation of C@CuS core–shell frameworks. This structure improves the overall conductivity of the composite and mitigates volume expansion during lithiation. The core–shell structure dramatically enhances the reaction kinetics, cyclic stability, and rate performance of the material.

2. Materials and Methods

2.1. Materials

The reagents utilized included acetylene black (C45, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, analytical reagent grade, AR, ≥99.5%), thioacetamide (TAA, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, analytical reagent grade, AR, ≥99.0%), copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, analytical reagent grade, AR, ≥99.5%), and ethanol (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China, analytical reagent grade, AR, ≥99.7%).

2.2. Preparation of CuS Nanoparticles and Nanostructured C@CuS Core–Shell Framework

The composite material was prepared using a liquid-phase precipitation method. First, 0.282 g of C45, along with 0.75 g of TAA and 1.205 g of Cu(NO₃)₂·3H₂O, were added to a mixed solution of water and ethanol (V1:V2 = 3:7), totaling 100 mL. Next, the solution was ultrasonically dispersed for 20 min, stirred for 60 min, transferred to a flat-bottomed flask and heated in a water bath at 70 °C for 3 h. After cooling to room temperature, the sample was washed three times, each time with deionized water and absolute alcohol, respectively. Finally, the sample was centrifuged and vacuum-dried at 60 °C for 12 h, and a powder consisting of the nanostructured C@CuS core–shell framework was obtained. The synthesis process for CuS nanoparticles is similar to that of C@CuS composites, the only difference being that C45 was not added.

2.3. Materials Characterization

The crystal structure of the samples was characterized by X-ray diffraction (XRD) in the 2θ range of 15–80° using a D2 PHASER Gen2 instrument (Bruker Corp., Billerica, MA, USA) with Cu Kα radiation. The microstructure and morphologies of CuS and C@CuS composites were examined using transmission electron microscopy (TEM, JEM-2010, JEOL Ltd., Tokyo, Japan) and scanning electron microscopy (HR-SEM, S-4800, Hitachi, Tokyo, Japan). Energy-dispersive spectroscopy (EDS) equipped with SEM was employed to investigate the element distribution. The Raman scattering spectra of the samples were obtained using a Renishaw spectrophotometer (Horiba, Kyoto, Japan) with a 532 nm laser.

2.4. Electrochemical Measurements

Coin cells (2032) were assembled in a glovebox. The slurry was prepared by adding 70 wt.% C@CuS, 20wt.% acetylene black and 10 wt.% carboxymethyl cellulose (CMC) into water. The slurry was then coated onto a copper foil using an automatic coating machine and dried in a vacuum oven at 80 °C for 12 h. Finally, the copper foil was transferred to the vacuum glove box. A lithium foil was used as the counter electrode, and a Celgard2500 membrane was used as the separator. The electrolyte solution was a mixture of 1 M LiPF6 in ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate (1:1:1 vol%). A CR2032 button-type battery assembled with these components was tested using a Xinwei tester ct-4008 produced by the Newell Electronic Company (Shenzhen, China) for constant current charge and discharge tests within the voltage range of 0.01–3 V. Cyclic voltammetry (CV) was measured using a CHI660E electrochemical workstation (Austin, TX, USA) with a voltage range of 0.01 V to 3 V and a scanning rate of 0.1 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was conducted also using a CHI660E electrochemical workstation with a frequency range from 0.01 Hz to 1000 kHz and a perturbation voltage of 0.005 V. In this process, the diameter of the pole piece is 14 mm and the average value of the mass of active substance carried on each pole piece is about 0.84 mg. The areal density of the active material on the electrode is about 0.55 mg/cm².

3. Results

3.1. XRD and Raman Spectroscopy Characterization

Figure 1a shows the XRD patterns of CuS nanoparticles and C@CuS nanocomposites. The diffraction peaks of CuS nanoparticles are consistent with hexagonal CuS (JCPDS No. 06-0464). No additional diffraction peaks were observed, indicating the high quality of the sample. The diffraction peaks of C@CuS nanocomposites can also be indexed as hexagonal CuS. The broad diffraction peak located at a 2θ angle of around 20 to 26 degrees corresponds to amorphous carbon. Raman spectroscopy was utilized to confirm the presence of carbon and CuS in the composite, as displayed in Figure 1b. In pristine CuS, a strong peak at 467 cm⁻¹ due to the S-S stretching vibrational mode and a weak peak at 267 cm⁻¹ originated from the Cu-S stretching vibrational mode [17,25]. In contrast, the Raman spectrum of the C@CuS composite exhibits two more peaks at 1377 cm⁻¹ and 1588 cm⁻¹, corresponding to the D band and G band of carbon, respectively [25,26].

3.2. Morphology and Structure Analysis

The morphology and microstructure of C45, CuS nanoparticles, and C@CuS composites were examined using TEM (Figure 2). Figure 2a clearly shows that C45 has a distinct skeleton structure composed of uniform and spherical carbon nanoparticles with a diameter of around 80 nm. Figure 2b displays the uniform CuS nanoparticles, with a diameter of around 20 nm. Figure 2c provides a detailed view of the composite structure, where the following components can be observed: a carbon skeleton as a white line, carbon nanoparticles (arrows a, b, c) and CuS nanoparticles (arrows e, f, g) growing on the surface of carbon spheres. To enhance the CuS coverage on the carbon framework, we can introduce surfactants, extend the mixing duration or increase the quantity of the raw materials. The high-resolution transmission electron microscopy (HRTEM) image presented in Figure 2d offers a clear visualization of the C@CuS composite structure. This image delineates the lattice fringes characteristic of the hexagonal crystal system of CuS, alongside the amorphous carbon particles, with a distinct interface between these two phases being prominently visible. An analysis of these lattice fringes revealed interplanar spacings (d-spacing) of 2.274 Å and 2.813 Å, which are indicative of the (006) and (103) crystallographic planes of the hexagonal CuS, respectively. Figure 2d shows the SAED patterns of the nanocomposites. The polycrystalline diffraction rings correspond to the (103), (006), and (008) crystal planes of CuS with a hexagonal structure. Figure 2e–h present the SEM images alongside the corresponding EDS elemental mapping for the C@CuS composite. These elemental mappings confirm a uniform distribution of Cu, S, and C, with the corresponding atomic ratios being 35.6%, 32.0% and 32.4%. These results confirm the successful synthesis of C@CuS core–shell composites.

3.3. Lithium Storage Mechanism

Cyclic voltammetry (CV) curves were obtained at a scanning rate of 0.1 mV s⁻¹ to investigate the lithium storage mechanism of the C@CuS nanocomposites and CuS nanoparticles (Figure 3a,b). Figure 3a shows that, during the first cathodic scan, a weak peak appears around 0.7 V and can be attributed to the formation of a solid electrolyte interface (SEI) film [17], which then disappears in subsequent cycles. Two distinct cathodic peaks (around 1.45 V and 1.96 V) appear, corresponding to the two-step reaction of CuS to Cu (Equations (1) and (2)) [27]. During the first anodic scan, two strong peaks at 2 V and 2.35 V can be observed, probably indicating the two-step transformation of Cu to CuS. In the following cycles, the CV curves of the C@CuS composite closely resemble those of pristine CuS, suggesting an improved cyclic stability in the composite. Compared to Figure 3a, Figure 3b shows similar reduction and oxidation peaks, with the exception of minor shifts in positions. This indicates that CuS nanoparticles and C@CuS nanocomposites have the same electrochemical lithium storage kinetics mechanism.

Equations:

2CuS + 2Li⁺ + 2e⁻ = Cu₂S + Li₂S

(1)

Cu₂S + 2Li⁺ + 2e⁻ = 2Cu + Li₂S

(2)

Discharging and charging curves at a current density of 50 mA g⁻¹ for CuS nanoparticles and C@CuS composite are shown in Figure 3c,d. The initial Coulombic efficiency of the C@CuS composite is 64.4%, with specific discharge and charge capacities of 867.4 and 558.9 mA g⁻¹, respectively. For CuS, the corresponding values are 68.9%, 1108.4 mA g⁻¹ (discharge), and 764.3 mA g⁻¹ (charge), respectively. The initial Coulombic efficiency of the C@CuS nanocomposite material is slightly lower than that of the CuS nanoparticles. The large initial irreversible capacity observed in both samples is due to the formation of SEI films [22]. Compared to CuS, the lower initial capacity of C@CuS can be attributed to C45, which has a lower theoretical specific capacity of 372 mA g⁻¹, lower than that of CuS. This lower theoretical capacity of the carbon component in the C@CuS composite contributes to the overall reduced initial capacity when the material is utilized in an LIB. Comparing Figure 3c,d, it can be seen that the C@CuS composite exhibits minimal polarization variation over the first three cycles, whereas the CuS nanoparticles experience significant polarization variations that gradually decrease over the same number of cycles. This may be due to the synergistic effect of the core–shell structure. On the one hand, the C45 core enhances the wetting ability of the composite with the electrolyte, affecting the initial contact and ion transfer while also reducing the lithium-ion concentration gradient within the active material, thus lowering concentration polarization. On the other hand, the C45 core with good flexibility and electronic conductivity enhances the electronic transport performance of the composite (Figure S1) while also alleviating the volume changes during the charge–discharge process, thus reducing the ohmic polarization between the active material and the current collector. In a nutshell, the core–shell structure improves the electronic and ionic transport properties of the composite, reducing polarization, which is also consistent with the improvement in rate performance and impedance performance tests.

3.4. Cycling Performance

The cycling performance of CuS nanoparticles and C@CuS composites at a current density of 100 mA g⁻¹ is depicted in Figure 4a. After 100 cycles, the specific capacity of CuS nanoparticles and C@CuS composites is 552 and 811 mAh g⁻¹, respectively, highlighting the superior reversible capacity of the C@CuS composite. Additionally, the capacity of the C@CuS composite gradually increases during cycling.

Figure 4b shows the long-term cycling performance of CuS and C@CuS composites at a current density of 1000 mA g⁻¹. After 800 cycles, the specific capacity of CuS nanoparticles is 346 mAh g⁻¹, while that of the C@CuS composite reaches 463 mAh g⁻¹. The capacity of the C@CuS composite continues to increase up to the 194th cycle, reaching a maximum capacity of 825 mAh g⁻¹, which later stabilizes at around 450 mAh g⁻¹ after 800 cycles. The increase in the late long-cycle capacity may be due to the release of electrode stress and structural changes in electrode material. On the one hand, during the initial charge–discharge cycle, the electrode material undergoes stress due to volume expansion. Composite materials with a flexible carbon core are beneficial for the absorption or gradual release of this stress, which enhances the lithium-ion diffusion performance of the electrode material. On the other hand, in the charge–discharge cycle, the decomposition of the CuS particles into smaller nanoparticles will provide more active centers for lithium-ion storage, which ultimately leads to a significant enhancement of the long-cycle performance of the composites at high current densities [25,26,28,29].

Although the initial Coulombic efficiency of the C@CuS nanocomposite material is slightly lower than that of the CuS nanoparticles, the composite material exhibits superior cycling stability and a specific capacity after long cycles. In other words, the composite material has demonstrated superior lithium storage performance overall.

A comparison of previously reported cycling properties of CuS-based electrodes is presented in

Table 1. Cycling performances of CuS-based electrodes as anode materials for LIBs.

Electrode Materials	Initial Discharge Capacity (mAh g−1)	Discharge Capacity (mAh g−1)	Current Density (mA g−1)	Refs.
CuS nanorods	735	390 (250 cycles)	112	[30]
CuS nanorods	550	472 (100 cycles)	100	[27]
CuS monolith	310	468 (100 cycles)	112	[16]
CuS/2.5%CNT	430	558 (100 cycles)	400	[23]
Vs-CuS/rGO	882	500 (100 cycles)	112	[24]
		513 (450 cycles)	500
CuS@S	790	513.2 (200 cycles)	100	[31]
		402 (200 cycles)	1120
CuS-GO	625	497 (100 cycles)	200	[19]
		348 (1000 cycles)	2000
CuS NWs/rGO	908	620 (100 cycles)	280	[32]
		320 (430 cycles)	2240
C@CuS	1108	811 (100 cycles)	100	this work
		463 (800 cycles)	1000

. It can be concluded that nanostructured composites with carbon materials (such as graphene and carbon nanotubes) play a crucial role in improving the reversible capacity of CuS, which aligns with our findings [15,18,19,23]. Based on this comparative study, we conclude that the nanostructured C@CuS core–shell framework synthesized through a simple one-step precipitation method exhibits favorable lithium-ion storage performance.

3.5. Rate Performance, Galvanostatic Charge–Discharge, and Ex Situ Analyses after Cycling

The rate performance of CuS nanoparticles and C@CuS composites at different current densities is shown in Figure 5a. At current densities of 100, 200, 500, 1000, 2000, 3000, and 5000 mA g⁻¹, the specific capacities of CuS are 740, 478, 314, 248, 178, 137, and 70 mAh g⁻¹, respectively. On the other hand, C@CuS composites exhibit higher reversible capacities of 605, 576, 533, 494, 437, 395, and 314 mAh g⁻¹, respectively, at the corresponding current densities. The rate performance of C@CuS composites outperforms that of CuS nanoparticles, which is particularly evident at high current densities (e.g., 5000 mA g⁻¹). At 5000 mA g⁻¹, the capacity of CuS decreases rapidly to 70 mAh g⁻¹, while the C@CuS composite retains a capacity of 314 mAh g⁻¹. This further confirms the excellent rate performance of the C@CuS composite. Notably, when the current density is decreased back to 100 mA g⁻¹, the capacity of the C@CuS composite recovers to 800 mAh g⁻¹, surpassing its initial capacity at this current density. This capacity increase phenomenon is consistent with the cycling performance shown in Figure 4a,b.

The charging and discharging curves of C@CuS composite and CuS nanoparticles at different current densities are displayed in Figure 5b,c, respectively. At an initial current density of 100 mA g⁻¹, the charging and discharging plateaus correspond to the oxidation and reduction peaks, which align with the CV curves (Figure 3a,b). However, as the current density increases, the charging and discharging plateaus gradually vanish, indicating the presence of a pseudocapacitive Li-ion storage mechanism in C@CuS composites [33]. The excellent rate performance is primarily attributed to the nanoscale core–shell framework. This carbon framework provides an effective electronic conductive network, ensuring an efficient electron transfer between the active material and the collector, thus enhancing the utilization rate of the active material [34,35,36,37]. The presence of a carbon framework provides several benefits in the context of CuS nanoparticles. It prevents the aggregation of CuS nanoparticles and mitigates the volume expansion that occurs during lithium intercalation/deintercalation processes [38,39,40,41].

Figure 6a,b show the SEM images of the composite electrode before and after 800 cycles at a current density of 1000 mA g⁻¹. It can be found that, after charging and discharging, the original nanoscale particulate matter became aggregates and a few microcracks with widths of roughly 50 nm and lengths of roughly 500 nm to 2 μm appeared, indicating the occurrence of some volume changes. Although the presence of aggregates causes microcracks in the electrode material, the original gap between the particles no longer exists, which greatly enhances the degree of close arrangement of aggregates and ion/electron transfer performance. The corresponding EDS images of the composite material are shown in Figure 6c–f, in which Cu, S and C elements are uniformly distributed on the electrode surface, respectively, indicating that the composite material still maintains the homogeneity of the components after 800 cycles. Upon comparison of Figure 6 with Supplementary Figure S2, it is observed that, after 800 cycles, CuS has developed larger and longer cracks (with the widest cracks reaching up to 500 nm) on the jelly-like substance and the homogeneity of the composition is not as good as that of C@CuS. It is well-known that crack formation is closely related to the volume change in electrode materials. This directly confirms that the incorporation of carbon and the construction of the C@CuS core–shell structure have mitigated the volume changes in CuS nanomaterials during the lithiation/delithiation process.

3.6. EIS Measurements

To further analyze the electrochemical properties, EIS measurements were conducted on the CuS nanoparticles and C@CuS electrodes before and after cycling, as shown in Figure 7. The impedance spectrum consists of a semicircle in the high-mid-frequency region and a slope line in the low-frequency region [42]. The semicircle represents the charge transfer resistance, while the slope line corresponds to a Warburg impedance associated with lithium-ion diffusion within the active material. Before cycling, the charge transfer resistance of the C@CuS composites is smaller than that of CuS. This trend remains after cycling. However, notable changes occur after three cycles: the charge transfer resistance of the C@CuS composite decreases, while that of CuS increases. This can be attributed to the decreased resistance of charge migration and the increased interfacial electrochemical reaction in the C@CuS composites [39].

The Li⁺ diffusion coefficient (D_Li⁺) is an important physical parameter that characterizes the ability of lithium-ions to diffuse within the electrode material in LIBs. In general, a larger D_Li⁺ represents a better high-current discharge capability and electrochemical performance. Based on EIS measurement data, the D_Li⁺ of the CuS and the C@CuS are 1.04 × 10⁻¹⁵ and 7.79 × 10⁻¹⁵ cm²/s, respectively, after cycling for three cycles. The D_Li⁺ of the C@CuS is seven times as high as that of CuS. In summary, the results of the EIS measurements align with the cycling performance and rate performance data, further evidencing the superior electrochemical characteristics of the C@CuS core–shell composite.

4. Conclusions

In this work, we have successfully synthesized a carbon-coated copper sulfide (C@CuS) core–shell framework utilizing a straightforward one-step precipitation method. The resulting composite material has demonstrated exceptional electrochemical performance, which can be attributed to the synergistic effects of the carbon core and CuS shell. The carbon core not only significantly improves the electrical conductivity of the active material but also effectively mitigates volume changes during the lithium intercalation and deintercalation processes. Furthermore, it prevents the agglomeration of CuS nanoparticles, thereby enhancing the overall electrochemical properties. The C@CuS core–shell composite has exhibited a remarkable cycle life, particularly at elevated current densities, along with superior rate performance. These characteristics make the C@CuS composite a highly promising anode material for lithium-ion batteries (LIBs), being especially suitable for applications requiring rapid charge/discharge capabilities and high energy storage. The innovative core–shell architecture and the simple synthesis approach presented in this study can be extended to the development of other metal sulfide-based materials. This offers a potential pathway to further improve the anode materials for next-generation LIBs. Our findings contribute to the advancement of LIB technology, providing a viable solution to meet the growing demands for high-performance energy storage systems.