Facile fabrication and electrochemical properties of high-quality reduced graphene oxide/cobalt sulfide composite as anode material for lithium-ion batteries

Abstract

A reduced graphene oxide (rGO)/cobalt sulfide composite is synthesized with a simple and efficient ultrasound-assisted wet chemical method. The morphology and microstructure of the composite are examined with field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results confirm that cobalt sulfide nanoparticles are homogeneously and tightly attached on the surfaces of rGO. As an anode material for lithium-ion batteries, this composite delivers a high reversible capacity of 994 mA h g⁻¹ after 150 cycles at a current density of 200 mA g⁻¹. A synergistic effect combining the merits of rGO and cobalt sulfide nanoparticles endows the composite with superior electrochemical performances over those of pure cobalt sulfide.

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1. Introduction

Lithium-ion batteries (LIBs) with high storage capacity and good cycling stability are of increasing demand for applications in portable electronic devices and hybrid electric vehicles.^1–3 In commercial LIBs, graphite is commonly employed as the anode material because of its flat potential profile versus lithium and its structural stability during cycling. However, the small theoretical capacity of graphite (372 mA h g⁻¹) necessitates the development of alternative anode materials.^4,5 Unlike graphite materials, inorganic nanomaterials, including metal oxides,^6–9 nitrides,^10,11 and chalcogenides,^12–15 are based on unusual conversion reaction mechanisms and some of them show high reversible capacities. Among these materials, cobalt sulfides in the form of various crystalline phases with a wide range of stoichiometries, including CoS, CoS₂, Co₃S₄, and Co₉S₈, and nonstoichiometric Co_1−xS have attracted much attention due to their high theoretical capacity, good thermal stability, natural abundance and environment friendliness.^16–18 However, like other conversion reaction-based anode materials, there are still many challenges on practical LIB application of cobalt sulfide. These include their poor rate capability, limited capacity and rapid capacity fading, caused by their low conductivity and drastic volume expansion during the discharge/charge process. In general, two widely used strategies have been developed to address these problems. One approach is to accommodate the volumes change with novel nanostructures.^19–21 The other effective way is to introduce carbonaceous materials, because the elastic feature of carbon supports can provide a cushion effect against volume strain. Meanwhile, the carbon materials can also improve the electrical conductivity of the cobalt sulfide anode materials.^22–24 As a versatile carbonaceous material, graphene is an excellent substrate for loading active nanomaterials for LIBs due to its outstanding electronic conductivity, high charge mobilities, good mechanical strength, structural stability and large specific surface areas.^25–27 It has been reported that the Li-storage performance of cobalt sulfide could be improved by combining with graphene.^28–31 For example, compared to pure CoS₂ with fast capacity fading after 10 cycles, a CoS₂/graphene composite retains a reversible capacity of 650 mA h g⁻¹ after 40 cycles at 50 mA g⁻¹.²⁸ In another work, a solvothermal method was reported to synthesize cobalt sulfides/graphene composite. LIB anode made from this composite still have a reversible capacity of 950 mA h g⁻¹ after 50 cycles at a current density of 100 mA g⁻¹.²⁹ Namely, graphene is able to increase the conductivity of cobalt sulfide and cushion its volume change stress during charge/discharge process. It is noteworthy that although remarkable progresses have been made, challenges such as the time consuming synthesis methods, poor connection between graphene and the cobalt sulfide materials, and low production rate, etc., still need to be addressed.

Here, a reduced graphene oxide (rGO)/cobalt sulfide composite was synthesized by a simple and efficient ultrasonic-assisted wet chemical method at low temperature. Cobalt sulfide nanoparticles are found to be tightly and homogeneously attached onto the surface of rGO forming an ultrathin sheet like structure. The composite shows excellent lithium storage properties with high capacity (about 994 mA h g⁻¹ after 150 discharge/charge cycles at a current density of 200 mA g⁻¹) and good rate capability.

2. Experimental

2.1 Synthesis of rGO/cobalt sulfide

Graphene oxide (GO) was synthesized from natural graphite powder using a modified Hummers' method (see the ESI†).^32,33 The rGO/cobalt sulfide composite was prepared with a simple and efficient route. In a typical process, 160 mL GO aqueous dispersion (0.25 mg mL⁻¹) was mixed with 35 μL hydrazine hydrate (80%) and 625 μL NH₃·H₂O (25%) via sonication. 2 mmol Co(CH₃COO)₂·4H₂O was dissolved in 40 mL deionized water and slowly added into the above homogeneous GO solution and then sonicated for another hour. Subsequently, freshly prepared Na₂S solution (1.0 g in 1 mL deionized water) was added dropwise under sonication. All reactions were carried out without external heating and the temperature during reactions is in the range of 45 to 50 °C. After 1 hour ultrasonic assisted reaction, the composite was filtrated, washed with deionized water and freeze-dried. Finally, the product was annealed in Ar/H₂ (95/5 v/v) gas at 400 °C for 2 hours. For comparison, pure cobalt sulfide was also prepared by a similar procedure in the absence of GO.

2.2 Characterization

Morphologies, structures, and chemical compositions of the samples were investigated with scanning electron microscopy (SEM, Philips FEG SEM XL30) and energy-dispersion X-ray spectroscopy (EDX), transmission electron microscopy (TEM, Philips CM200 FEG TEM operated at 200 kV), atomic force microscopy (AFM, VEECO Nanoscope IIIa), X-ray diffraction (XRD, Philips X'Pert diffractometer with Cu Kα radiation), Raman spectroscopy (Renishaw 2000 laser Raman microscope equipped with a 514 nm argon ion laser), and X-ray photoelectron spectroscopy (XPS, VG ESCALAB 220i-XL surface analysis system, using a monochromated Al Kα X-ray gun with photon energy of 1486.6 eV).

2.3 Electrochemical measurements

Electrochemical characterizations were measured with CR2032 coin-type cells assembled in a dry argon-filled glove box (MBRAUN, MB 200B) with an oxygen concentration below 1 ppm. The working electrodes were prepared by mixing the composite, carbon black, and sodium carboxylmethyl cellulose (CMC) with a weight ratio of 70 : 20 : 10 and pasting onto a copper foil. The test cell consists of a working electrode and a lithium counter electrode, which were separated by polypropylene membrane (Celgard 2400, Inc., USA). The electrolyte is a solution of 1 M LiPF₆ in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1 : 1 in volume). Galvanostatic discharge/charge experiments were carried out with a MACCOR 4000 battery test system over a voltage window of 0.01–3.0 V (vs. Li⁺/Li). Cycle voltammetry (CV) was measured using a CHI 660D electrochemical workstation. Electrochemical impedance spectroscopy (EIS) measurements were performed with a ZAHNER-elektrik IM6 impedance measurement unit over a frequency range of 0.01–10⁵ Hz with an AC amplitude of 5 mV.

3. Results and discussion

AFM morphological image and the corresponding cross-section analysis of GO on the silicon substrate demonstrate that the average thickness of GO sheets is approximately 1 nm (Fig. S1 in ESI†), indicating single-layered GO sheets have been obtained.^31,34 XRD pattern of sample obtained before adding Na₂S is shown in Fig. S2.† The diffraction peaks can be indexed to Co(OH)₂ and CoOOH. As cobalt acetate salt is added to the GO dispersion, the Co²⁺ ions coordinate to the negatively charged GO layers and henceforth act as nucleation centers for the formation of Co(OH)₂ and CoOOH nanocrystals. After ultrasonically treat for 1 hour, Co(OH)₂ and CoOOH are uniformly formed on the surfaces of rGO (Fig. S2b†). Upon addition of Na₂S, the Co(OH)₂ and CoOOH nanocrystals attached to the rGO layers are converted into cobalt sulfide nanocrystals.³⁵ Under this condition, the GO is simultaneously reduced to rGO. Finally, the product was annealing at 400 °C in Ar/H₂ (95 : 5 vol. ratio) for 2 hours to obtain the final composite material.

The crystalline phase of cobalt sulfide in the prepared composite was determined by XRD. As shown in Fig. 1a, the XRD pattern of the composites indicate that it contains mainly cubic Co₉S₈ (ICDD PDF #86-2273) belonging to the Fm3̄m space group (no. 225). Additionally, diffraction peaks originated from Co_1−xS (ICDD PDF #42-0826) can also be observed. For pure cobalt sulfide prepared in the absence of GO, all of the diffraction peaks can be indexed to the Co₉S₈ phase with relative poor crystallization.

Fig. 1 (a) XRD pattern of the prepared pure cobalt sulfide and rGO/cobalt sulfide composite, (b) Raman spectrum of the pure cobalt sulfide and the rGO/cobalt sulfide composite, (c) XPS survey spectrum, (d) Co 2p and (e) S 2p core level spectra of the prepared rGO/cobalt sulfide composite.

It is well known that Raman spectroscopy is a powerful technique to study carbonaceous materials. Here, Raman spectroscopy is used to confirm the existence of rGO in the as-prepared composite. Fig. 1b shows typical Raman spectra of the pure cobalt sulfide and the rGO/cobalt sulfide composite. Compared to the spectrum of pure cobalt sulfide, in the spectrum of the rGO/cobalt sulfide composite, two peaks at 1353 and 1584 cm⁻¹ appear, corresponding to the breathing modes of rings or κ-point phonons of A_1g symmetry (D band) and the E_2g phonon of sp²-bonded carbon atoms in a two-dimensional hexagonal lattice (G band), respectively, which suggests the existence of rGO.

XPS was used to provide further evidence for the successful combination of cobalt sulfide with rGO, and to examine chemical state of cobalt and sulfur in the composite. Fig. 1c shows a survey spectrum, indicating the composite consists of C, O, S and Co. A Co 2p_2/3 core level spectrum is shown in Fig. 1d. The two main peaks at 778.4 and 781.0 eV can respectively be attributed to Co–S and Co–O bonds, respectively.^36–38 The Co–O bond was derived from the coordination between the unstable surface cobalt ions and oxygen atoms from the rGO.^36,39 Besides, a shake-up satellite can be found at about 785.0 eV. The peaks between 790 and 805 eV are the corresponding Co 2p_1/2 signals of their Co 2p_3/2 counterparts. For the S_2p core level spectrum (Fig. 1e), the peaks located at 161.6 and 162.5 eV are attributed to sulfide (S²⁻) and metal deficient sulfide (S₂²⁻) species, respectively. The peaks at 163.3 and 164.3 eV correspond to polysulfides (S_n²⁻).^40–43

Morphologies and microstructures of the as-prepared samples were investigated by SEM and TEM. Fig. 2a is an SEM image of the rGO showing characteristic wrinkled thin veil-like structure with many folds at the edges (the detailed characterization and analysis of the rGO are listed in Fig. S3 in ESI†). For the rGO/cobalt sulfide composite, the morphology exhibits a loosely packed structure consisting of some sheet-like building units inheriting from rGO (Fig. 2b). Such a structure provides easy access of the electrolyte to the interfaces of the electrode material. In contrast, the pure cobalt sulfide shows a more compact structure than that of the composite (Fig. S4 in ESI†), suggesting the introduction of rGO facilitates formation of loose structure.

Fig. 2 SEM images of (a) rGO and (b) rGO/cobalt sulfide composite, (c) A TEM and (d) an HRTEM images of the rGO/cobalt sulfide composite, (e) an SAED pattern of the composite, and (f) EDS elemental mapping of the composite.

A TEM image (Fig. 2c) of the rGO/cobalt sulfide composite reveals that cobalt sulfide nanoparticles are uniformly attached on the rGO sheets. An HRTEM image (Fig. 2d) shows lattice fringe spacings of about 0.49 and 0.22 nm, which correspond respectively to spacings of the (200) and the (420) planes of the Co₉S₈ phase. In addition, a corresponding SAED pattern of the composite (Fig. 2e) shows the polycrystalline nature of the Co₉S₈. The diffraction rings can be readily indexed to the (311), (400), (440), (511) and (800) planes of the Co₉S₈ phase, which is in a good agreement with the XRD analysis. Fig. 2f shows EDS mappings of the rGO/cobalt sulfide composite. It is clearly shown that the elements of C, S, and Co are homogenously distributed, indicating uniform coating of cobalt sulfide on the rGO surfaces.

CV and EIS tests were performed with coin-type test cells to study the electrochemical reactions in the composite during the lithiation/delithiation processes. Fig. 3a shows typical CV curves of the prepared rGO/cobalt sulfide composite anode for the first three cycles over a voltage window of 0.01–3.0 V (vs. Li⁺/Li) at a scanning rate of 0.1 mV s⁻¹. It can be seen that the CV profiles show typical characteristics of cobalt sulfide based anodes.^24,28 In the first cycle, a broad and weak cathodic peak at about 0.68 V is ascribed to the decomposition of electrolyte and the formation of solid electrolyte interface (SEI) layer. The sharp cathodic peak at 0.91 V corresponds to the reduction of Co ion to Co⁰ (Co₉S₈ + 16Li⁺ + 16e⁻ ↔ 8Li₂S + 9Co). Such a peak is also observed, although significantly shifts to 1.3 V in the subsequent cycles, which is in good agreement with the literature.^24,44 The lithiation voltage in the following cycles is higher than that in the first cycle, probably due to the improved kinetics of the rGO/cobalt sulfide anode resulting from a microstructure alteration after the first lithiation process. This phenomenon commonly appears in other conversion reaction-based anode materials.^45,46 The main oxidation peak at about 2.07 V could be ascribed to the oxidation of Co⁰ to Co ion in the delithiation process. The CV curves of rGO/cobalt sulfide composite are almost overlapped after the 1^st cycle, and show high symmetry shapes, indicating the reversible electrochemical reactions of the composite.²⁸ Furthermore, the microstructure alteration of the prepared composite after the first cycle is characterized by XRD. As shown in Fig. S5,† no peaks except Cu substrate are observed after the first cycle, indicating the microstructure was totally changed at low potentials. This phenomenon is in good agreement with that of CoS₂ anode material.⁴⁷ EIS curves of the electrodes for the as-prepared composite and the pure cobalt sulfide were shown in Fig. 3b. It can be seen that each of the Nyquist plots consists of a depressed semicircle in the medium-frequency region and a linear part in the low-frequency region. The semicircle is attributed to the charge-transfer reaction at the electrolyte/electrode interface. The subsequent straight line represents typical Warburg resistance, which is attributed to the diffusion of lithium ions in the bulk of the active anode materials. Clearly, the diameter of the semicircle of the rGO/cobalt sulfide composite electrode is much smaller than that of the pure cobalt sulfide, which means a higher electrochemical activity of the rGO/cobalt sulfide composite electrode, resulting in high capacity and good rate capability.²⁴ The decrease of resistance might be attributed to the high conductivity of the rGO.

Fig. 3 (a) CV curves of the rGO/cobalt sulfide composite at a scan rate of 0.1 mV s⁻¹ and (b) EIS of the rGO/cobalt sulfide composite and pure cobalt sulfide electrodes.

Performance of the as-prepared rGO/cobalt sulfide composite in battery was further studied by using a galvanostatic discharge/charge technique. Fig. 4a shows the 1^st to 150^th discharge/charge cycle of the composite at a current density of 200 mA g⁻¹ over a voltage window of 0.01–3.0 V (vs. Li⁺/Li). The initial discharge and reversible charge capacities of the composite anode are 990 and 733 mA h g⁻¹, respectively. The initial Coulombic efficiency is about 74%, comparable to or even higher than those of recently reported cobalt-based composites.^24,28–30 The irreversible capacity loss in the 1^st cycle is attributed to the electrolyte decomposition and the formation of solid electrolyte interface (SEI) film on the surface of electrode. During the 2^nd cycle, the discharge capacity decreases to 748 mA h g⁻¹ with a corresponding charge capacity of 713 mA h g⁻¹, showing a much higher Coulombic efficiency than the 1^st cycle. Fig. 4b shows cycling stability of the pure cobalt sulfide and rGO/cobalt sulfide composite at a current density of 200 mA g⁻¹. After 150 discharge/charge cycles, the composite anode shows a high reversible capacity of 994 mA h g⁻¹ with a Coulombic efficiency over 97%. This capacity value is even higher than the initial discharge capacity, and is much higher than that of the control battery using a pure cobalt sulfide anode (320 mA h g⁻¹ after 100 cycles). Notably, this reversible capacity is higher than the theoretical capacity of cobalt sulfide (Co₉S₈: 539 mA h g⁻¹). Such a high reversible capacity indicates high accessibility for lithium ion insertion and extraction in this material and the excess over the theoretical value probably arises from interfacial lithium storage on the composite surfaces.⁴⁵ The good cycling performance might be ascribed to the homogeneous integration of the cobalt sulfide with the elastic rGO, which can effectively accommodate the physical strains caused by the large volume changes during the charge/discharge process.⁴⁸ It should be noted that the capacity increases continuously as increasing the cycling numbers. The increased capacity could be attributed to the destruction in the graphite lattice during the lithiation/delithiation and the generation of more defects and active sites for lithium ion storage^49,50 and the reversible growth of the polymeric gel-like film by the kinetically activated electrolyte degradation, which has also been observed in previous literatures.^28,51,52

Fig. 4 (a) Discharge and charge voltage profiles of the rGO/cobalt sulfide composite, (b) Cycling performance of the prepared pure cobalt sulfide and rGO/cobalt sulfide electrode cycled at a current density of 200 mA g⁻¹, and corresponding Coulombic efficiency of rGO/cobalt sulfide electrode, (c) and (d) Rate-capability performance of the rGO/cobalt sulfide composite at various current densities of 100, 250, 500, 1000, and 2000 mA g⁻¹.

Good rate performances are desirable for developing LIBs with high power densities. The rate capability of the as-prepared composite was evaluated at various current densities. Fig. 4c and d show the rate capability of the prepared composite over a voltage window of 0.01–3.0 V. The discharge capacities are about 724, 678, 651, 607, and 547 mA h g⁻¹ at the current densities of 100, 250, 500, 1000, and 2000 mA g⁻¹, respectively. Up to 76% discharge capacity is retained from 100 mA g⁻¹ to 2000 mA g⁻¹. Moreover, after changing the current density from 2000 mA g⁻¹ to 100 mA g⁻¹, the discharge capacity first recovers to 690 mA h g⁻¹ and increases to 892 mA h g⁻¹ after another 40 discharge/charge cycles, showing its excellent reversibility. In contrast, the pure cobalt sulfide exhibits much worse rate performance (560 mA h g⁻¹ after 30 cycles, Fig. S6 in ESI†). For the conversion reaction-based materials, the poor cycling stability has always been a bottleneck for practical application. Compared with the recent studies on LIB with cobalt sulfide/graphene composite anodes (Table S1 in ESI†), the composite in this work possesses good capacity retention and a high capacity. The improvement in electrochemical Li-ion storage performance of the as-prepared rGO/cobalt sulfide composite is attributed to the following factors: (i) the thin particle-sheet structure of the rGO/cobalt sulfide electrode which greatly decrease the inter-particle resistance of electron pathways, shortens the diffusion length for both electrons and Li ions, and facilitates the transport of electrons in the bulk electrode during the discharging/charging; and (ii) the elastic rGO matrix not only acts as the support for cobalt sulfide and provides high electrical conductivity pathway, but also accommodates the volume changes and prevents the breakdown of the overall electrode during the cycling process.^53,54

4. Conclusions

In summary, we have developed a simple but efficient ultrasonic-assisted wet chemical method to fabricate uniform cobalt sulfide nanocrystals on rGO. The as-prepared rGO/cobalt sulfide composite shows much better electrochemical performance including higher reversible capacity and rate capacity than pure cobalt sulfide material in LIB applications. The improvements in capacity, cycling stability and rate capability can be attributed to the synergetic effect between the small cobalt sulfide nanoparticles and conductive rGO matrix. It is envisaged that the present method may be adaptable for the large-scale production of metal sulfide/rGO composites for next generation energy transfer and storage device applications.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06067a

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