In situ formation of flower-like CuCo₂S₄ nanosheets/graphene composites with enhanced lithium storage properties

Abstract

Flower-like CuCo₂S₄ nanosheets/graphene composites (abbreviated as CCS–G) were prepared by using a one-pot hydrothermal method. The morphology and structure of CCS–G were investigated by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and Raman spectroscopy. SEM and TEM images showed the hierarchical structure of CCS–G with flower-like morphology, which were composed of many nanosheets. As an anode material for the rechargeable lithium-ion batteries (LIBs), CCS–G exhibited the remarkably enhanced Li-storage performance in comparison with the pristine flower-like CuCo₂S₄ nanosheets (abbreviated as CCS). The reversible capacity of CCS–G is maintained to be 778.9 mA h g⁻¹ at a current density 0.1C after 60 cycles. Even at a high current density of 1C, CCS–G retains a much high specific capacity of 866.3 mA h g⁻¹ after 500 cycles.

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1. Introduction

Transition metal sulfides have attracted tremendous attention due to their unique physical and chemical properties,^1–9 especially excellent electrical performance for lithium-ion batteries (LIBs), sodium-ion batteries and supercapacitors.^3,10,11 Among these transition metal sulfides, Cu-based and Co-based sulfides are the most popular ones, owing to their variable stoichiometric compositions including Cu₂S,^12,13 CuS,^14,15 CoS,¹⁶ CoS₂,^17–20 Co₉S₈,²¹ etc. On the one hand, it's generally recognized that introduction of Cu or Co into a metallic sulfide would greatly improve the conductivity and provide a buffer matrix during lithiation/delithiation reactions.²² On the other hand, CoS₂ has a high theoretical lithium storage capacity of 870 mA h g⁻¹, which is larger than that of the mostly used graphite (372 mA h g⁻¹) in commercial LIBs.²³ Meanwhile, CuS has a high electrical conductivity of 10⁻³ S cm⁻¹.^14,24,25 These merits have aroused extensive interests of researchers when they are used as electrode materials. Therefore, ternary Cu–Co sulfide is believed to be a promising anode material for LIBs.

Similar to transition metal oxides as anode materials, the spinel-type transition metal sulfides generally suffer from poor electronic conductivities, and large volume expansions during discharge/charge process, which leads to a rapid fading of cycling performance and therefore greatly restricts their commercial applications. To deal with these issues, two approaches have been applied to enhance the electrochemical activity of sulfide materials. One approach is to develop nanomaterials with tailored morphologies and sizes for the sake of high surface-to-volume ratios and high freedoms for the volume change during Li⁺ insertion/extraction process and other reactions.^{6,9,19,25–27} Among those structures, the flower-like nanosheets may be favorable for Li⁺ transportation and electrolyte penetration.^28–32 The other approach is to composite transition metal sulfides with graphene or other carbonaceous materials.^19,33–37 In the composites, graphene and other carbonaceous materials not only act as the conductive network but also serve as the buffer layer for the volume expansion during the cycling.^12,21,38,39 Compared with other carbonaceous sources, graphene as a two-dimensional carbon material behaves higher conductivity, larger specific surface area, better flexibility and more superior mechanical properties.⁴⁰ Also, the graphene layers can effectively reduce the dissolution of Li₂S_x into the electrolyte. Previous studies have shown that synergistic effect between graphitized carbon materials and the metal sulfides, which contributes to the improved electrode kinetics and cycling stability.^41–44 Therefore, the composites of metal sulfides and graphene were expected to provide high reversible capacities, good cycling stabilities and superior rate capabilities.

Herein, copper had been combined with cobalt to fabricate the flower-like CuCo₂S₄ nanosheets (abbreviated as CCS) without any templates, which were further hybridized with graphene to form the flower-like CuCo₂S₄ nanosheets/graphene composites (abbreviated as CCS–G). When evaluated as electrode materials for LIBs, CCS–G showed higher reversible discharge/charge specific capacities, more excellent cycling stabilities and better rate capabilities than CCS.

2. Experimental

2.1 Preparation of materials

All the chemicals were of analytical grade. In a typical procedure, 30 mg of graphene oxide (GO; XFNANO, China) was dispersed into 30 mL of ethylene glycol (EG) by ultrasonic treatment for 30 min. Separately, 1 mmol of CuCl₂·2H₂O, 2 mmol of CoCl₂·6H₂O and 4 mmol of (NH₂)₂CS were dissolved in 40 mL of EG. Then, the GO dispersion was added into the above solution under magnetic stirring. Subsequently, the mixture was transferred into a Teflon-lined stainless steel autoclave (100 mL), which was heated to 200 °C for 12 h. Finally, the resulting black product was collected by centrifuging and washing with absolute ethanol for several times. Dried powders were obtained by heating at 60 °C under vacuum overnight. CCS was synthesized as same as CCS–G except no GO was added into the mixture.

2.2 Material characterizations

X-ray diffraction (XRD) patterns were recorded by a MaXima 7000 diffractometer (Shimadzu, Japan) with Cu Kα radiation (40 kV, 100 mA). Raman spectra were taken from a Invia Reflex spectrometer (Renishaw, UK) with an excitation wavelength of 532 nm. Morphologies and chemical compositions were characterized by scanning electron microscope (SEM) (JEOL JSM-6510LV, Japan), transmission electron microscope (TEM) (JEOL JEM-2100, Japan), as well as the energy dispersive X-ray spectroscopy (EDAX) (Oxford Instrument). Thermogravimetric analysis (TGA) was carried out by Q600-SDT (TA Instruments, USA) apparatus at a heating rate of 10 °C min⁻¹ in a flowing air to determine the amount of graphene in the samples. Nitrogen-adsorption isotherms were measured by Micrometrics ASAP 2020 adsorption analyzer at 77.4 K.

2.3 Electrochemical measurements

To measure the electrical conductivity, CCS and CCS–G powders were pressed into pellets, 15 mm in diameter and 1–1.5 mm in thickness. A good electrical contact was made by coating silver paste on both contact surfaces of the pellets. The conductivity was calculated according to the I–V curves recorded by an electrochemical workstation CHI 660B (Shanghai, China).

The typical electrodes were fabricated by mixing 70 wt% CCS–G or CCS, 20 wt% carbon black (Super-P-Li), 10 wt% polymer binder of sodium carboxymethyl cellulose (CMC). The diameter of the electrodes was 1.5 cm. The electrodes were assembled into CR2032 coin-type cells with Li foil as the counter electrode. The electrolyte was 1 M LiPF₆ dissolved in a mixture of ethylene carbonate and diethyl carbon (EC/DEC; 1 : 1 v/v). The discharge–charge cycling of the cells were galvanostatically investigated in the voltage range of 0.005–3 V (Land CT2001A, China). Cyclic voltammetry (CV) measurements were carried out in the voltage range between 0.005 and 3 V at a scan rate of 0.1 mV s⁻¹ using CHI 660B (Shanghai, China). Electrochemical impedance measurements (EIS) was performed on an electrochemical workstation CHI 660D (Shanghai, China) in the frequency range from 0.1 to 1.0 × 10⁵ Hz.

3. Results and discussions

The representative XRD patterns of CCS and CCS–G were shown in Fig. 1a. The diffraction peaks can be indexed to the (111), (022), (113), (004), (224), (115) and (044) planes of the cubic CuCo₂S₄ phase (JCPDS card no. 42-1450). One peak marked with asterisk can be assigned to the cubic CoS₂ phases (JCPDS card no. 41-1471). Meanwhile, no obvious peak at 24.5° can be observed from CCS–G. Due to the hybridization of graphene with CCS, the diffraction feature of graphene is greatly weakened.²⁴ In spite of this, the presence of graphene in the composite can be evidenced by Raman spectrum. As shown in Fig. 1b, Raman scatterings at 1354 and 1585 cm⁻¹ correspond to D-band and G-band, respectively.²⁴ Considering the intensity ratio of I_D/I_G, CCS–G exhibits an increased value of 0.98, larger than the value of CCS (0.82). The increased intensity ratio is owing to the formation of new and smaller sp² domains, which indicates GO reduction during the hydrothermal process.^{14,34–36,44} The peaks centered at 474, 509 and 660 cm⁻¹ are observed from Raman spectra of CCS–G and CCS, which are characteristics of CuCo₂S₄ and may be attributed to the stretching S–S mode,^14,25,45 and E_g mode of Co.⁴⁶

Fig. 1 (a) XRD patterns of CCS and CCS–G; the peak marked with asterisks can be assigned to cubic CoS₂ phase. (b) Raman spectra of GO, CCS and CCS–G.

Scanning electron microscope (SEM) is used to characterize surface morphologies of the as-prepared CCS and CCS–G. As seen in Fig. 2a and b, CCS is assembled by a large number of single nanosheets. There are some slices raised around the surface, and the flower-like nanosheets possess three dimensional channels in the interior. Meanwhile, some irregular hierarchic structures can be found in some areas of the CCS images. This phenomenon may be related to the increased numbers of crystal embryos at the beginning of nucleation. The average sizes of CCS are estimated to be ∼5 μm. Fig. 2c and d display the morphology of CCS–G, which are similar to CCS. But, the semi-transparent graphene nanosheets are clearly visible (as indicated by arrows). These morphologies suggest that graphene nanosheets do not wrap CCS and restrict its growth.

Fig. 2 SEM images of (a and b) CCS and (c and d) CCS–G.

The representative transmission electron microscope (TEM) images of CCS–G are shown in Fig. 3a and b. It is clearly observed that CCS were hybridized with graphene (as indicated by the arrows). Fig. 3c exhibits the inter-planar spacing of 0.17, 0.28 and 0.54 nm, which are consistent with the (044), (113) and (111) planes of CCS–G. Selected area electron diffraction (SAED) pattern of CCS–G in Fig. 3d also depicts facets of (113) and (044). In brief, results of HRTEM reveal the cubic CuCo₂S₄ phase and existence of the flower-like CuCo₂S₄ nanosheets/graphene composites.

Fig. 3 (a and b) TEM images, (c) HRTEM image, and (d) SAED pattern of CCS–G.

The energy-dispersive X-ray spectroscopy (EDAX) analysis of CCS–G is shown in Fig. 4a. Four elements (Cu, Co, S and C) are identified from the full surveys of EDAX spectrum. The atomic ratio of Cu/Co/S is calculated to be 1.00 : 1.86 : 3.58, which is close to the chemical stoichiometry of CuCo₂S₄. Elemental mapping analysis of CCS–G in Fig. 4b demonstrates that Cu, Co, S and C elements are uniformly distributed in the sample of CCS–G.

Fig. 4 (a) EDAX spectrum and (b) SEM image and the corresponding elemental mappings (copper, cobalt, sulfur, and carbon) of CCS–G.

Fig. 5a depicts the representative thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) curves of CCS–G. A prominent weight loss of ∼10% from 500 to 600 °C in the TGA curve and a strong peak centered at ∼560 °C in the DTG curve are observed, which correspond to the weight loss of carbon from graphene in CCS–G. The measured weight percentage of graphene in CCS–G is slightly larger than the feed ratio of ∼9%, which results from a reduced weight of CCS due to the incomplete reaction. In the DTG curve, the weight loss below 500 °C can be attributed to the surface and inherent moisture release below 200 °C, the thermal decomposition of CCS from 200 to 425 °C, as well as the thermal decomposition of Co₂S at ∼480 °C.^47,48 There is an obvious discrepancy of carbon content between EDAX and TGA analyses. The high content of 37.73% detected by EDAX may be due to that the samples are supported by the conductive carbon tapes. Therefore, the actual content of carbon in CCS–G can be ∼10% measured by TGA.

Fig. 5 CCS–G: (a) TGA and DTG curves. (b) N₂ adsorption–desorption isotherm; the inset shows the BJH pore size distribution.

The nitrogen sorption isotherm of CCS–G in Fig. 5b is type IV, with a small hysteresis loop in the relative pressure range of 0.6–0.95, indicating a mesoporous structure.⁴⁹ The BET specific surface area, the total pore volume and the average BJH pore diameter are determined to be 23.8 m² g⁻¹, 0.05 cm³ g⁻¹ and 12.1 nm, respectively. During the discharge/charge cycles, the mesoporous structure of CCS–G can facilitate lithium ion transportation and increase the electrolyte diffusion to active sites, which is one of the typical factors to improve the electrochemical performance of LIB anodes.⁴⁹

At room temperature, the electrical conductivities of CCS and CCS–G are determined to be 2.71 × 10⁻⁶ S cm⁻¹ and 8.43 × 10⁻⁴ S cm⁻¹, respectively. As expected, the hybridization of graphene into CCS could lead to a considerable increase in electrical conductivity, which reached 310 times in this work.

Electrochemical performances of CCS–G and CCS are tested by cyclic voltammetry (CV). As shown in Fig. 6a, the first cycle of CCS has two reduction peaks and two oxidation peaks. The cathodic peak at 1.2 V is indicative of the reduction of Co(III) or Co(II) to metal Co, and the weak peak at 1.0 V is related to the formation of solid electrolyte inter phase (SEI) and decomposition of electrolyte.²¹ The anodic peaks at 2.0 and 2.3 V are assigned to oxidation of Li₂S into sulfur meanwhile formation of Li-ions.⁵⁰ From the second cycle on, the reduced peak at ∼1.2 V moves to ∼1.3 V, and the weak peaks at 1.6, 1.8 and 2.1 V appear. The peaks at ∼1.3 V and 2.1 V are assigned to the lithiation process of Cu(II) and Co(II) or Co(III);^18,24 the peaks at 1.6 V and 1.8 V may be owing to decomposition of lithiated electrode to stable phase Cu metal, Co metal and Li₂S.^34,50 The positions of anodic peaks from the second cycle on are in agreement with the first cycle. It can be clearly observed that the peak intensities decrease from the first to fifth cycle, which indicates the poor cycling stability. Fig. 6b shows CV curves of CCS–G, in which positions of the reduction and oxidation peaks are similar to CCS. But, the peak intensities from the second to fifth cycle coincide well, which firmly proves that CCS–G possess a better cycling stability than CCS.

Fig. 6 CV curves of (a) CCS and (b) CCS–G in a voltage scope of 0.005–3.0 V at a scan rate of 0.1 mV s⁻¹.

Fig. 7a and b shows the initial 1, 2, 5, 25 and 60 discharge–charge voltage profiles of CCS and CCS–G at a current density of 0.1C (1C = 1000 mA g⁻¹) within a voltage window of 0.005–3.0 V. In Fig. 7a for CCS, there are two voltage plateaus in the initial discharge (1.0 and 1.2 V) and charge (2.0 and 2.3 V) cycle. These plateaus correspond to the lithiation and delithiation of CCS electrodes. The plateau at 1.0 V can be attributed to the formation of SEI films in the first lithiation. We can observe that the voltage plateau at 1.0 V disappears in the second and subsequent discharge curves. It is ascribed to the electrochemical reaction changes of the internal batteries.²⁴ From the 25^th cycle on, only one plateau is seen, which reflects the capacity fading in the subsequent cycles. As shown in Fig. 7b, the voltage plateaus of CCS–G are similar to CCS in Fig. 7a. However, the fading trend of CCS–G is apparently slower than that of CCS, which reveals the better cycling stability of CCS–G.

Fig. 7 Discharge–charge profiles of (a) CCS and (b) CCS–G at a current density of 100 mA g⁻¹. Cycling performances of (c) 0.1C and (d) 1C (the first 3 cycles are still 0.1C) of CCS and CCS–G.

In the initial cycle at a current density of 0.1C (Fig. 7c), CCS and CCS–G deliver the specific discharge capacities of 796.6 and 1004.4 mA h g⁻¹, and the specific charge capacities of 712.3 and 956.7 mA h g⁻¹, respectively. After 60 cycles, the discharge capacity of CCS is 498.8 mA h g⁻¹, but CCS–G still retain a discharge capacity of 778.9 mA h g⁻¹. Obviously, the cycling performance of CCS–G is better than that of CCS.

The long-term cycling properties of CCS and CCS–G at a high rate of 1C are shown in Fig. 7d. To protect the cells, the first three cycles still run at a rate of 0.1C. From the fourth cycle on, the current density is raised to 1C. After 500 cycles, the discharge capacity of CCS is only 226.7 mA h g⁻¹, while CCS–G retains a much higher discharge capacity of 866.3 mA h g⁻¹. It is worth noting that the specific capacities of CCS–G fade to 376.1 mA h g⁻¹ at initial 81 cycles and then rise to 1046.5 mA h g⁻¹ from 82^nd to 324^th cycles. This capacity recovery may be related to the pseudo capacity effect of the polymeric/gel films, which has been reported by many groups.^{11,13,14,17,51,52}

The rate capabilities of CCS and CCS–G are shown in Fig. 8. The final reversible capacities of CCS–G at 0.1, 0.2, 0.5, 1 and 2C are 980, 1014, 902, 693 and 404 mA h g⁻¹, respectively. When the discharge/charge current densities keep at 0.1C, 0.2C or 0.5C, the specific capacities of CCS–G retain well. But, they are fading rapidly when the densities increase to 1C or 2C. Once the current density returns to 0.1C, the final reversible capacity of CCS–G recovers to 846 mA h g⁻¹. Undoubtedly, the rate performances of CCS–G are superior to that of CCS, which demonstrates that compositing with graphene is a promising approach to largely enhance the lithium storage properties of anode materials for LIBs.

Fig. 8 Rate performances of CCS and CCS–G at various current densities.

In order to study kinetics of the electrodes during Li⁺ discharge–charge process, electrochemical impedance measurements (EIS) is employed as an effective approach. Fig. 9 displays the typical Nyquist plots with the common feature of a high- to medium-frequency depressed semicircle followed by a linear tail in the low frequency region. The size of the semicircular that encompasses the high- to medium-frequency response is an indication of the charge-transfer resistance (R_ct) in the electrode reaction. At the open circuit voltage (Fig. 9a), R_ct of CCS and CCS–G are 150.2 and 50.4 Ω, respectively. When cycled for 5 cycles by CV, R_ct of CCS and CCS–G decrease to 40.2 Ω and 21.5 Ω, respectively. Regardless of the fresh or cycled cells, R_ct of CCS–G is always smaller than CCS. It indicates that the improved electrochemical reaction dynamics is attributed to the introduction of graphene because graphene can enhance the electron transfer during the discharge/charge cycling. Therefore, CCS–G exhibits a high reversible capacity, excellent cyclic stability, and high-rate capability.

Fig. 9 Nyquist plots of CCS and CCS–G (a) at the open circuit voltage and (b) cycled for 5 cycles by CV.

4. Conclusions

In summary, the flower-like CuCo₂S₄ nanosheets/graphene composites have been synthesized by a one-pot hydrothermal method. The flower-like CuCo₂S₄ nanosheets with a size of ∼5 μm are composed by lots of inter-connected nanosheets. The reversible capacity of CCS–G is maintained to be 778.9 mA h g⁻¹ at a current density 0.1C after 60 cycles. Even at a high current density of 1C, CCS–G retains a much high specific capacity of 866.3 mA h g⁻¹ after 500 cycles. These results manifests that CCS–G possesses excellent electrochemical performances and may be a promising anode material for LIBs.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51402242, 21275119, 51473136, 21575116), and the Cultural Program for Young Talents of Science and Technology in Innovating New Products from CQ CSTC (CSTC2013KJRC-QNRC50001).

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