Superior Electrochemical Performance of Two-Dimensional RGO/Cu/Cu₂O Composite as Anode Material for Lithium-Ion Batteries

Abstract

In recent years, graphene has attracted the interest of many researchers working on LIB anode materials owing to its unique 2D structure, thermal stability, and fast electron transfer. In this work, RGO/Cu/Cu₂O nanocomposites were synthesized through a hydrothermal procedure. The as-prepared nanocomposites exhibited a high lithium storage capacity with improved cycling stability and great rate performance, i.e., the discharge capacity was 371.8 mAh/g after 100 cycles at a current density of 500 mA/g. These excellent properties were associated with the sheet structure symmetry of graphene enriched with the multifunctional Cu-Cu₂O component, which prevented aggregation and accommodated the volume changes of the anode material during the charge–discharge tests. The RGO/Cu/Cu₂O composite conferred to the LIB anode the ability to resist electrode cracking. The approach proposed in this paper can be also generalized for the synthesis of other carbon-based anode materials for LIBs.

1. Introduction

Nowadays, it is important to design low-cost, high-energy sources that are eco-friendly to minimize the need for energy related to promoting industrial growth. Extensive use of fossil energy has caused environmental damage; therefore, lithium-ion batteries (LIBs) are being widely used to power vehicles and energy-storage devices [1,2,3]. The 3D transition series metal oxides (MxOy, where M is Ni or Fe) have improved cyclic performance and very high reversible capacity; therefore, they are considered to be prospective candidates for LIBs as anode materials [4,5]. Among them, binary oxides (e.g., Co₃O₄/CuO [6,7], NiO/Co₃O₄ [7], Fe₂O₃/CuO [8], and CuO/SnO₂ [9]) have been reported to have many benefits. In particular, the nanocomposite Cu₂O-CuO is used in various applications, such as nanoreactors [10], drug delivery [11], supercapacitors [12], and rechargeable batteries [13]. Among the above-mentioned materials, cuprous oxide (Cu₂O) is the most important owing to its non-toxicity, low cost, high electrocatalytic activity, and suitable redox potential. Significant consideration has been devoted to the fabrication of Cu₂O electrode materials for glucose sensors, which in turn revealed substantial electrocatalytic activities [14,15,16]. Considering the low electrical conductivity of Cu₂O, combination with copper metal with high catalytic activity, better electrical conductivity, and low cost appears promising [17,18]. In fact, by anchoring Cu nanoparticles to the copper oxide surface, a synergistic effect provides the composite anode material with good catalytic activity and conductivity. Luo et al. reported an electrochemical deposition method for the preparation of nanocomposite Cu₂O/Cu that revealed great electrochemical activity when used as a sensor for the detection of hydrogen peroxide [19]. Furthermore, the Cu-Cu₂O composite has been studied for application in lithium storage devices [20,21,22,23,24]. In order to further improve the electric conductivity, reduced graphene oxide (RGO) can be added to the composite. Owing to the excellent chemical and physical properties of RGO, its composite-based materials have been used for improving electrochemical applications [25,26]. For example, a paper reported the synthesis of Cu₂O-CuO-RGO composite to improve cyclic performance in supercapacitors [27]. Furthermore, hollow CuO-Cu₂O nanospheres/graphene composites prepared by using a microwave-assisted method have also shown significant stability for use in LIBs [28].

Herein, we designed a new two-dimensional RGO/Cu/Cu₂O composite prepared in ethylene glycol (EG). Owing to its synergetic effect, the transfer of charge carriers between Cu and Cu₂O, and interfacial polarization generated by multiple interfaces in the nanocomposite, the two-dimensional RGO/Cu/Cu₂O composite exhibited outstanding electrochemical properties when tested for LIBs.

2. Materials and Methods

2.1. Material Preparation

Analytical-grade reagents were used for this study before additional purification. Hummer’s method was applied for the preparation of GO. The composite was prepared by adding a molar ratio of copper chloride to GO and a molar ratio of NaOH to GO. NaOH (300 mg) was added to the ethylene glycol (20 mL) for the preparation of the stock solution, heated for 1 h at 120 °C under an inert atmosphere, and the temperature was lowered to 80 °C. In total, 45 mg of GO was added to 30 mL of EG for 1 h under ultrasonication, and then copper chloride (170 mg) was added with continuous stirring for 1 h. Using an inert flow with continuous stirring, the mixture was further heated to 220 °C for 40 min. A definite quantity of the solution was added rapidly to the mixture. The obtained mixture was continuously further heated at 220 °C. To ensure the completion of the reaction, the reaction time was fixed at 2 h. The Cu-RGO composite was thoroughly washed with ethanol, filtered by using suction filtration, and vacuum dried at 45 °C for 22 h. Finally, the Cu-RGO composite was heated at 200 °C in a furnace for 4 h to obtain the Cu-Cu₂O-RGO composite.

2.2. Material Characterization

The structural and morphological features of the as-prepared nanocomposite were studied by X-ray diffraction, scanning electron microscopy (SEM), and transmission microscopy (TEM). Raman spectroscopy was used to determine the defects in the material. Spectra were obtained with a Renishaw RM1000 confocal microscope. X-ray photoelectron spectroscopy was employed for the determination of the atomic concentrations on the sample’s surface and chemical binding energies by using monochromated Al Kα radiation.

2.3. Electrochemical Characterization

Coin cells were employed for the measurement of the electrochemical performance of the as-synthesized product, which contained lithium foil and an active material as the counter and working electrodes, respectively. The positive electrode was prepared by using the slurry coating method, consisting of the electrode material (0.07 g), acetylene black (0.015 g), and poly (vinylidene difluoride, 0.015 g) binder with a weight ratio of 70:15:15 on Ni-foam in N-methyl-2-pyrrolidone solvent. The working electrode was attained by vacuum drying at 120 °C for 20 h. A glove box filled with inert gas was used to assemble the testing cells in order to limit the oxygen and moisture concentrations to below 1 ppm. A solution containing 1 M LiPF₆ dissolved in ethylene carbonate/dimethyl carbonate was used as the electrolyte. Discharge/charge measurements of the testing cells were performed with a potential window between 0.01 and 3.0 V for different current densities. The electrochemical workstation was used for CV and EIS measurements.

3. Results

3.1. Process of RGO/Cu/Cu₂O Fabrication

Scheme 1 displays the overall synthesis procedure of the two-dimensional RGO/Cu/Cu₂O composite material. On the surface of GO, the oxygenic functional groups adsorbed the copper (Cu²⁺) ions with the help of EG that acted as a reducing agent. The Cu nanoparticles were constantly developed in situ on the nanosheets of RGO, leading to high loading and good dispersion. After calcination in air at 200 °C, Cu₂O nanoparticles were formed and immobilized within the layers of graphene sheets. Finally, a sheet-structured two-dimensional RGO/Cu/Cu₂O material with a small particle size was facilely prepared.

3.2. Crystal Structure and Morphology

The XRD spectra for the graphite oxide (GO) and RGO/Cu/Cu₂O composite are depicted in Figure 1. An extensive peak is located at around 25°, which is consistent with the (002) reflection of the GO. Prominent peaks for Cu and Cu₂O can be observed in the XRD spectra for the RGO/Cu/Cu₂O composite. No other impurities were found in the samples.

The Raman spectra of the RGO/Cu/Cu₂O composite and GO are displayed in Figure 2. The two broad peaks observed at 1340 and 1580 cm⁻¹ originated from the GO and RGO/Cu/Cu₂O composite and were recognized as the different bands, i.e., D and G, respectively. In general, useful information about the crystallinity degree of different carbon materials is closely related to the ratio of both bands (ID/IG) [29]. As compared to GO, the RGO/Cu/Cu₂O composite showed a difference in the intensity ratio I_D/I_G. The intensity ratio value for GO was 0.90, while that of the RGO/Cu/Cu₂O composite was 1.23. Owing to the higher ID/IG value for the RGO/Cu/Cu₂O composite, the degree of crystallinity was better, which can be attributed to the materialization of different and lesser sp² dominions during reduction compared with the GO [30].

To investigate the morphology of the RGO/Cu/Cu₂O composite, SEM was used. Figure 3a shows the average diameter of the multicomponent Cu/Cu₂O, which was approximately 20 nm. It can be observed that RGO exhibited a sheet structure (Figure 3b). In Figure 3c, a uniform distribution of Cu/Cu₂O NPs on the surface of the RGO sheets was obtained. In addition, Figure 3d,e reveal that Cu/Cu₂O NPs are highly dispersed on the surface of graphene and completely covered the graphene sheets which would provide an easy path for Li ions diffusion and to accommodate the volume change effect during cycling. Furthermore, one could easily observe that multicomponent Cu/Cu₂O were well decorated on sheet to sheet structure of graphene (Figure 3f) [31].

The TEM analyses reported in Figure 4a,b confirm that the RGO sheets were well covered by Cu/Cu₂O with a diameter of approximately 10–20 nm. The nanosheets of RGO are expected to act as a conductive linkage for the multicomponent Cu/Cu₂O NPs, favoring a good distribution of Cu/Cu₂O NPs on individual graphene sheets. In addition, the Cu/Cu₂O NPs were wrapped in the RGO sheets, as shown in Figure 4c. TEM (Figure 4d,e) reveal that individual RGO sheets are homogenously covered with Cu/Cu₂O NPs of uniform diameter. Furthermore, Figure 4f support the SEM findings. No obvious uncovered portion as well as no significant odd size particles can be found.

3.3. XPS Analysis

The chemical composition of the as-prepared RGO/Cu/Cu2O composite was studied by using XPS (Figure 5a,c). The survey spectrum (Figure 5a) indicates the presence of O, C, and Cu. XPS was used to determine the chemical composition of the composite (Figure 5a,c). Figure 5a shows the survey spectrum where the presence of O, C, and Cu is evidenced. The Cu 2p spectrum shown in Figure 5b exhibits two main peaks at 933.5 eV and 953.4 eV relating to Cu 2p_3/2 and Cu 2p_1/2, respectively [32]. This result corroborates the identification of the products of the reduction reaction of Cu (II) as Cu and Cu₂O, while no CuO was found. In the C 1s spectrum (Figure 5c), three components were observed corresponding to the non-oxygenated carbon at 285.9 eV (79.2 %), C-O at 286.4 eV (14.8 %), and C=O at 288.8 eV (6.0 %). Comparing these results with previously reported XPS spectra relating to GO [33], it can be observed that the distribution of the components linked with functional oxygenated groups noticeably decreased, showing that the GO in our sample was reduced.

3.4. Electrochemical Studies

The first three cycles of the charge–discharge curves relating to the RGO/Cu/Cu₂O composite electrode are displayed in Figure 6a. The electrochemical investigations were conducted at a high current density of 500 mA/g between 0.02 and 3.0 V (vs Li⁺/Li). The initial charge–discharge capacities were found to be 856.3 and 517.7 mAh/g, respectively. The failure of irreversible capacity can be related to various phenomena, such as lithium interfacial storage, the growth of SEI films, and electrolyte decomposition [34,35]. The multiple plateaus observed for the discharge curves of the RGO/Cu/Cu₂O composite can be attributed to the ion transport kinetics of the metal oxide.

The cyclic performance of the RGO/Cu/Cu₂O composite at a 500 mA/g current density between 0.01 and 3.0 V is shown in Figure 6b. A reversible capacity of 369.0 mAh/g was retained by the composite after 100 cycles, even at a very high current density of 500 mA/g. The RGO/Cu/Cu₂O composite showed no obvious capacity loss, providing great stability. It was observed that the composite exhibited an increase in the reversible capacity from 70 to 100 cycles. The irreversible capacity increase might have been due to the following three reasons: first, the multi-components of the composite were activated during the cycling process; second, few oxygenic groups present on the sheets would react with Cu, leading to an enhancement in the conductivity; and third, a small part of the SEI layer was decomposed. The rate performance of the RGO/Cu/Cu₂O composite is presented in Figure 6c. The RGO/Cu/Cu₂O composite electrode allowed the LIB to deliver very high capacities of 537.8, 430.6, 327.7, 255.7 and 456.5 mAh/g, at various current densities of 300, 500, 800, 1000, and 300 mA/g, respectively. It is worth mentioning that a 456.5 mAh/g capacity was achieved when the current density was retuned to 300 mA/g. The RGO/Cu/Cu₂O composite electrode not only showed improved cycling performance, but also good performance and reversible cycling, even at a very high current density, i.e., 500 mA/g, which can be ascribed to the conductive network of RGO sheets.

The first three CV curves of the RGO/Cu/Cu₂O composite are shown in Figure 6d. The first reduction peak at 1.15 V can be attributed to the reduction of Cu^I into Cu metal, while the formation of the SEI layer is indicated by the reduction peak at 0.69 V. The decomposition of SEI was shown at 1.59 V (oxidation peak) during the charging process. The peak at 2.45 V was due to the regeneration of Cu₂O by the reaction of Cu⁰ and Li₂O. The shape of the CV curve began to change at the second cycle; the second reduction peak was similar to previously reported data relating to Cu₂O [36]. The cathodic peaks at 0.88 and 1.54 V show the SEI layer formation and the reduction of Cu₂O to Cu(0). In addition, the considerable shift of the peak at 2.45 V to a high voltage is associated with electrode polarization.

To further understand the advantages of RGO/Cu/Cu₂O nanocomposites, an EIS test was performed. A Nyquist plot and equivalent model circuit (inset) of the composite after 20 cycles at 500 mA/g are presented in Figure 6e. The semicircles lie in both low- and high-frequency regions. As already reported, the region of low frequencies related to the diffusion of Li+ ions in the electrode material, and the high-frequency zone was attributed to resistance in the SEI and charge transfer resistance at the interface of electrode material. As clearly observed in the inset of Figure 6e, the solution resistance (R₁) and charge resistance (R₂) consisted of electronic impedance (Rt). The plots in the low-frequency region were attributed to Warburg impedance (Z_w).

One of the most important parameters regarding electrochemical measurements is the lithium-ion diffusion co-efficient (D_Li). The D_Li can be determined by the Nyquist plots (Figure 6f) using the following equations:

$$Z=R_{ct}+R_s+\sigma w^{-1/2}$$

(1)

$$D_{Li}=\frac{R^2{T}^2}{2A^2{n}^4{F}^4{C}_{Li}^2{\sigma }^2}$$

(2)

The obtained D_Li value for the RGO/Cu/Cu₂O composite was 3.14 × 10⁻¹⁴ cm²/s. This very high value indicates the great ability of the material to trap Li⁺ ions. This finding corroborates previous results, indicating that the RGO/Cu/Cu₂O composite exhibits excellent electrochemical performance, reversible capacity, and good Coulombic efficiency.

4. Conclusions

RGO/Cu/Cu₂O nanocomposites were fabricated by a facile hydrothermal approach. The nanoparticles had diameters of 10–20 nm and exhibited outstanding electrochemical performance in terms of their reversible capacity, capacity retention, and rate performance. The results show that the performance was mainly related to the synergistic role of both the Cu-Cu₂O nanoparticles and RGO sheets. The Cu-Cu₂O nanoparticles on the RGO nanosheets formed a sheet structure that minimized the chance of the stacking of RGO sheets. RGO nanosheets hindered the dilatation volume of the Cu-Cu₂O nanoparticles by acting as effective 2D conductive networks during the charging and discharging processes.