One-Pot Synthesis and High Electrochemical Performance of CuS/Cu1.8S Nanocomposites as Anodes for Lithium-Ion Batteries

CuS and Cu1.8S have been investigated respectively as anodes of lithium-ion batteries because of their abundant resources, no environment pollution, good electrical conductivity, and a stable discharge voltage plateau. In this work, CuS/Cu1.8S nanocomposites were firstly prepared simultaneously by the one-pot synthesis method at a relatively higher reaction temperature 200 °C. The CuS/Cu1.8S nanocomposites anodes exhibited a high initial discharge capacity, an excellent reversible rate capability, and remarkable cycle stability at a high current density, which could be due to the nano-size of the CuS/Cu1.8S nanocomposites and the assistance of Cu1.8S. The high electrochemical performance of the CuS/Cu1.8S nanocomposites indicated that the CuxS nanomaterials will be a potential lithium-ion battery anode.


Structure and Morphology of CuS/Cu1.8S Nanocomposites
Scanning electron microscope (SEM, GeminiSEM300, Zeiss, Oberkochen, Germany) and X-ray diffraction (XRD, SmartLab, Rigku, Tokyo, Japan) were used to characterize the structure, morphology, and composition of the samples. The XRD measurements were performed in the rage of 20° to 80° at a measuring rate of 3°/min using a Cu Kα radiation.

Lithium-Ion Battery Performance of CuS/Cu1.8S Nanocomposites
The working anodes were mixed with as-prepared black powders, carbon black, and carboxymethyl cellulose (CMC) dissolved in deionized water with a weight ratio of 7:2:1. The obtained slurry was evenly coated on a copper foil and dried in a vacuum oven at 70 °C for 12 h. And then, the obtained coated foil was punched into disks with a diameter of 12 mm. The weight of each disk sheet was measured to calculate the mass of active materials. The mass density of the active material was calculated at about 0.4 mg cm −2 .
The electrochemical measurements were carried out on CR-2032 coin-type cells. The half-cells were assembled in an argon glove box. The concentrations of oxygen and moisture in the argon glove were lower than 1 ppm. The Celgard 2250 film and lithium metal disk were selected as diaphragm and counter electrode, respectively. The electrolyte was an organic electrolyte of 1 M LiPF6, which is dissolved in a mixture of dimethyl ethyl carbonate (DEC) and ethyl carbonate (EC) with a volume ratio of 1:1.
Land-ct2001A battery measuring system was used to test the charge-discharge characteristics under different current densities with a potential range of 0.01 V-3.0 V. CHI660E electrochemical workstation was used to get the cyclic voltammetry (CV) measurements with a scanning rate of 0.1 mV s −1 and in a potential range between 0.01 V and 3.0 V. The electrochemical impedance spectroscopy (EIS) was also obtained by the CHI660E electrochemical workstation with a frequency range of 10 −2 Hz-10 5 Hz. All the cells measured were set more than 24 h to ensure total penetration of the electrolyte into the diaphragm, and all measurement results were obtained at room temperature. Figure 2a shows the XRD patterns of our samples, which is in good agreement with the standard cards of PDF No. 06-0464 (CuS) and No. 24

Structure and Morphology of CuS/Cu 1.8 S Nanocomposites
Scanning electron microscope (SEM, GeminiSEM300, Zeiss, Oberkochen, Germany) and X-ray diffraction (XRD, SmartLab, Rigku, Tokyo, Japan) were used to characterize the structure, morphology, and composition of the samples. The XRD measurements were performed in the rage of 20 • to 80 • at a measuring rate of 3 • /min using a Cu Kα radiation.

Lithium-Ion Battery Performance of CuS/Cu 1.8 S Nanocomposites
The working anodes were mixed with as-prepared black powders, carbon black, and carboxymethyl cellulose (CMC) dissolved in deionized water with a weight ratio of 7:2:1. The obtained slurry was evenly coated on a copper foil and dried in a vacuum oven at 70 • C for 12 h. And then, the obtained coated foil was punched into disks with a diameter of 12 mm. The weight of each disk sheet was measured to calculate the mass of active materials. The mass density of the active material was calculated at about 0.4 mg cm −2 .
The electrochemical measurements were carried out on CR-2032 coin-type cells. The half-cells were assembled in an argon glove box. The concentrations of oxygen and moisture in the argon glove were lower than 1 ppm. The Celgard 2250 film and lithium metal disk were selected as diaphragm and counter electrode, respectively. The electrolyte was an organic electrolyte of 1 M LiPF6, which is dissolved in a mixture of dimethyl ethyl carbonate (DEC) and ethyl carbonate (EC) with a volume ratio of 1:1.
Land-ct2001A battery measuring system was used to test the charge-discharge characteristics under different current densities with a potential range of 0.01 V-3.0 V. CHI660E electrochemical workstation was used to get the cyclic voltammetry (CV) measurements with a scanning rate of 0.1 mV s −1 and in a potential range between 0.01 V and 3.0 V. The electrochemical impedance spectroscopy (EIS) was also obtained by the CHI660E electrochemical workstation with a frequency range of 10 −2 Hz-10 5 Hz. All the cells measured were set more than 24 h to ensure total penetration of the electrolyte into the diaphragm, and all measurement results were obtained at room temperature.   [30,51,52]. The peak of the (101) crystal plane of CuS and the (111) crystal plane of Cu 1.8 S could overlap to 27.8 • . The average sizes of crystal particles can be obtained by the Scherrer Equation [32,54]:

Results and Discussion
where λ is the wavelength of the X-ray applied in the measurement, k is a constant fact 0.9, θ is the diffraction angle, and b is the full width at half maximum (FWHM). The sizes of CuS/Cu 1.8 S particles were estimated to be 23 nm (D (110) ) and 22 nm (D (220) ), which are nearly equal to each other.
where λ is the wavelength of the X-ray applied in the measurement, k is a constant fact 0.9, θ is the diffraction angle, and b is the full width at half maximum (FWHM). The sizes of CuS/Cu1.8S particles were estimated to be 23 nm (D(110)) and 22 nm (D(220)), which are nearly equal to each other. The morphology of as-prepared samples was further investigated using SEM, as is shown in Figure 2b. It can be seen from the SEM image that the as-prepared samples consisted of nanoparticles with sizes in the range of 10 nm-100 nm, which was consistent with the results calculated by the Scherrer equation. The mole ratio of CuS and Cu1.8S was measured by Energy Dispersive Spectrometer (EDS). The EDS results are shown in Figure S1 and Table S1 of supplementary data, and the mole percentage of CuS is about 88%.

Electrochemical Performance of CuS/Cu1.8S Nanocomposites
In order to further understand the electrochemical process of CuS/Cu1.8S nanocomposites, the cyclic voltammetry (CV) curves were measured with a scanning rate of 0.1 mV s −1 and in a potential range between 0.01 V and 3.0 V. As shown in Figure 3a, during the first cathodic sweep (lithiation), two prominent reduction peaks at 2.0 V and 1.6 V were observed, which could be attributed to the process of CuS to LixCuS and the conversion of LixCuS to Cu and Li2S, respectively. During the first anodic sweep (delithiation), two obvious oxidation peaks at 1.9 V and 2.4 V were observed, which denoted the reversible process related to the cathodic reactions [31,32,34,47,48,54]. The reduction peak at 2.0 V transferred to 2.1 V in the second scan and faded in the subsequent scans, which has also been observed in other reports [32,47,55]. Furthermore, the reduction peak at 1.6 V transferred to lower potential values gradually, which could indicate the increase of energy and the polarization of the electrodes [55]. Figure 3b shows the corresponding initial five galvanostatic discharge-charge curves of CuS/Cu1.8S nanocomposites at 267 mA g −1 . In the first discharge process, two potential plateaus appeared at about 2.1 V and 1.6 V, and in the first charge process, two potential plateaus appeared at around 1.9 V and 2.3 V. These are matched with the redox peaks (redox reactions) of the first CV curve very well [47,48,54,55]. Compared with the CV curves, the discharge potential plateau at 2.1 V also gradually disappeared in the subsequent cycles. Importantly, we obtained the relatively The morphology of as-prepared samples was further investigated using SEM, as is shown in Figure 2b. It can be seen from the SEM image that the as-prepared samples consisted of nanoparticles with sizes in the range of 10 nm-100 nm, which was consistent with the results calculated by the Scherrer equation. The mole ratio of CuS and Cu 1.8 S was measured by Energy Dispersive Spectrometer (EDS). The EDS results are shown in Figure S1 and Table S1 of supplementary data, and the mole percentage of CuS is about 88%.

Electrochemical Performance of CuS/Cu 1.8 S Nanocomposites
In order to further understand the electrochemical process of CuS/Cu 1.8 S nanocomposites, the cyclic voltammetry (CV) curves were measured with a scanning rate of 0.1 mV s −1 and in a potential range between 0.01 V and 3.0 V. As shown in Figure 3a, during the first cathodic sweep (lithiation), two prominent reduction peaks at 2.0 V and 1.6 V were observed, which could be attributed to the process of CuS to Li x CuS and the conversion of Li x CuS to Cu and Li 2 S, respectively. During the first anodic sweep (delithiation), two obvious oxidation peaks at 1.9 V and 2.4 V were observed, which denoted the reversible process related to the cathodic reactions [31,32,34,47,48,54]. The reduction peak at 2.0 V transferred to 2.1 V in the second scan and faded in the subsequent scans, which has also been observed in other reports [32,47,55]. Furthermore, the reduction peak at 1.6 V transferred to lower potential values gradually, which could indicate the increase of energy and the polarization of the electrodes [55]. Figure 3b shows the corresponding initial five galvanostatic discharge-charge curves of CuS/Cu 1.8 S nanocomposites at 267 mA g −1 . In the first discharge process, two potential plateaus appeared at about 2.1 V and 1.6 V, and in the first charge process, two potential plateaus appeared at around 1.9 V and 2.3 V. These are matched with the redox peaks (redox reactions) of the first CV curve very well [47,48,54,55]. Compared with the CV curves, the discharge potential plateau at 2.1 V also gradually disappeared in the subsequent cycles. Importantly, we obtained the relatively higher initial discharge capability of 1130 mAh g −1 and charge capability of 707 mAh g −1 [29][30][31][32]47,48,54,55], which could be ascribed to the formation of solid electrolyte interface (SEI) and the existence of Cu 1.8 S [38][39][40][41][42][43][44]. The discharge capacities of the fourth and fifth cycles were about 610 mAh g −1 and 580 mAh g −1 , which are also higher than the theoretical capability 560 mAh g −1 of CuS. This extra capacity has been widely observed transition metal compounds [56], which can be attributed to the formation/decomposition of polymeric gel-like films around the transition metal particles [57], the interface lithium storage [58,59], and the surface conversion of LiOH to Li2O and LiH [60]. In the following cycles, the curves tended to overlap, which showed the outstanding cyclic stability of the CuS/Cu 1.8 S nanocomposites.
Materials 2020, 13, x FOR PEER REVIEW 5 of 12 higher initial discharge capability of 1130 mAh g −1 and charge capability of 707 mAh g −1 [29][30][31][32]47,48,54,55], which could be ascribed to the formation of solid electrolyte interface (SEI) and the existence of Cu1.8S [38][39][40][41][42][43][44]. The discharge capacities of the fourth and fifth cycles were about 610 mAh g −1 and 580 mAh g −1 , which are also higher than the theoretical capability 560 mAh g −1 of CuS. This extra capacity has been widely observed transition metal compounds [56], which can be attributed to the formation/decomposition of polymeric gel-like films around the transition metal particles [57], the interface lithium storage [58,59], and the surface conversion of LiOH to Li2O and LiH [60]. In the following cycles, the curves tended to overlap, which showed the outstanding cyclic stability of the CuS/Cu1.8S nanocomposites. Figure 4a depicts the cycling performance of the CuS/Cu1.8S nanocomposites at 267 mA g −1 . As shown in Figure 4a, the initial coulombic efficiency (CE) was 62.5% and increased to around 99% rapidly. At the same time, the specific capacity decreased from 1130 mAh g −1 to about 450 mAh g −1 .
The values of specific capacity and the corresponding coulombic efficiency almost maintained to 1000 cycles with a small range of fluctuation, which demonstrated excellent cycling stability and relatively high lithium storage capacity. A fluctuation of the capacity can be seen in Figure 4a. It is found that the specific capacity of some copper sulfide materials gradually increases with the charge-discharge process due to a possible activation process in the electrode materials [34,37,39,40,48,54]. In the experiments, the particle size ranges from 10-100 nm. The inside of the larger particles may not contribute to the capacity. With the charge-discharge process, the larger particles may decompose into smaller particles, which will provide more specific surface area and more effective active materials. In addition to the changes of the measuring temperature [32], the increased specific surface area and effective active materials may cause the fluctuation of specific capacity.  As shown in Figure 4a, the initial coulombic efficiency (CE) was 62.5% and increased to around 99% rapidly. At the same time, the specific capacity decreased from 1130 mAh g −1 to about 450 mAh g −1 .
The values of specific capacity and the corresponding coulombic efficiency almost maintained to 1000 cycles with a small range of fluctuation, which demonstrated excellent cycling stability and relatively high lithium storage capacity. A fluctuation of the capacity can be seen in Figure 4a. It is found that the specific capacity of some copper sulfide materials gradually increases with the charge-discharge process due to a possible activation process in the electrode materials [34,37,39,40,48,54]. In the experiments, the particle size ranges from 10-100 nm. The inside of the larger particles may not contribute to the capacity. With the charge-discharge process, the larger particles may decompose into smaller particles, which will provide more specific surface area and more effective active materials. In addition to the changes of the measuring temperature [32], the increased specific surface area and effective active materials may cause the fluctuation of specific capacity.
The electrochemical performances of CuS-based electrodes as anode materials for lithium-ion batteries were listed in Table 1. Compared with the previously reported results, the CuS/Cu 1.8 S nanocomposites in our work has good lithium-storage capability, high initial capacity, and good cycling stability. The CuS/Cu 1.8 S nanocomposites electrode could maintain the reversible capacity of 440 mAh g −1 after 100 cycles, 450 mAh g −1 after 500 cycles, 470 mAh g −1 after 700 cycles, and about 325 mAh g −1 after 1000 cycles. During the 1000 cycles, the average discharge capability and charge capability were 425 mAh g −1 and 420 mAh g −1 respectively. The high electrochemical performances of CuS/Cu 1.8 S nanocomposites electrode in our work could be attributed to the nano-size of the composites and the assistance of Cu 1.8 S [29][30][31][32][33][34]44,45,49,50]. The electrochemical performances of CuS-based electrodes as anode materials for lithium-ion batteries were listed in Table 1. Compared with the previously reported results, the CuS/Cu1.8S nanocomposites in our work has good lithium-storage capability, high initial capacity, and good cycling stability. The CuS/Cu1.8S nanocomposites electrode could maintain the reversible capacity of 440 mAh g −1 after 100 cycles, 450 mAh g −1 after 500 cycles, 470 mAh g −1 after 700 cycles, and about 325 mAh g −1 after 1000 cycles. During the 1000 cycles, the average discharge capability and charge capability were 425 mAh g −1 and 420 mAh g −1 respectively. The high electrochemical performances of CuS/Cu1.8S nanocomposites electrode in our work could be attributed to the nano-size of the composites and the assistance of Cu1.8S [29][30][31][32][33][34]44,45,49,50].    [28] As shown in Figure 4b, the rate capability of CuS/Cu 1.8 S nanocomposites anodes was further tested at a series of current densities. Despite an obvious fading of capacity during the initial several cycles, the CuS/Cu 1.8 S nanocomposites electrodes exhibited extraordinary stability and reversible capabilities in subsequent cycles. The reversible capacities had an obvious stepwise trend with the changing of current densities. The capacities were 1555 mAh g −1 , 485 mAh g −1 , 275 mAh g −1 , and 195 mAh g −1 at 100 mA g −1 , 200 mA g −1 , 500 mA g −1 , and 800 mA g −1 , respectively. When the current densities came back to 200 mA g −1 and 100 mA g −1 , the capacities could recover to 470 mA g −1 and 530 mA g −1 , respectively, which implied the outstanding stability and reversibility [32][33][34][36][37][38]48,54,55,61].
In order to obtain further understanding of the enhanced electrochemical performance of CuS/Cu 1.8 S nanocomposites electrode, electrochemical impedance spectroscopy (EIS) was measured. The Nyquist plot (black dots) measured before cycling and the relative fitting line (red line) were shown in Figure 4c. And the corresponding equivalent circuit was also showed in Figure 4c. Two depressed semicircles and a straight line were found in the Nyquist plot. The intercept in the high frequency region represents the ohmic resistance (R s ) of the electrode and electrolyte [61][62][63]. The small semicircle at high frequency is related to the impedance of SEI film (R cf ) [61,64]. The semicircle at high-medium frequency is related to the charge-transfer resistance (R ct ) induced by the diffusion of lithium ions between electrode and electrolyte. The slope of the straight line in low frequency corresponds to Warburg impedance (Z w ) relating to the diffusion of lithium ions in CuS/Cu 1.8 S nanocomposites electrode [44,47,48,52,54,55,61,62]. The Nyquist plot could be well fitted by the equivalent circuit shown in Figure 4c. The values of R s , R cf and R ct are 1.8 ohm, 34.3 ohm and 175 ohm, respectively. The EIS measured after 1000 cycles and the fitting line were shown in Figure S2. The values of R s , R cf and R ct increased after 1000 discharge-charge cycles, and the comparisons of R s , R cf and R ct before cycling and after 1000 cycles can be seen in Table S2. Furthermore, the diffusion coefficient of Li-ions (D Li + ) could be obtained by the Equations as follows [61,62]: where ω is the angular frequency in the low frequency region, σ is the Warburg coefficient, C is the concentration of lithium ions, n is the number of transferred electrons, F is the Faraday constant, T is the measuring temperature, A is the surface area of the electrode, and R is the gas constant [61,62]. The value of σ was obtained by linear fitting of Z real versus ω −1/2 Equation (3), and then the diffusion coefficient of Li-ions (D Li + = 3.89 × 10 −12 cm 2 S −1 ) was calculated by Equation (2) In addition, as shown in Figure 5, we presented a photographic image which is a test of a CuS/Cu 1.8 S nanocomposites half-cell after 1000 cycles illuminating an electronic watch and a LED lamp. The test demonstrated the high electrochemical performance and potential application of Cu x S nanomaterials in lithium-ion batteries.
In addition, as shown in Figure 5, we presented a photographic image which is a test of a CuS/Cu1.8S nanocomposites half-cell after 1000 cycles illuminating an electronic watch and a LED lamp. The test demonstrated the high electrochemical performance and potential application of CuxS nanomaterials in lithium-ion batteries.

Conclusions
In this work, CuS/Cu1.8S nanocomposites were firstly prepared simultaneously by the one-pot Synthesis method at a relatively high reaction temperature 200 °C. The CuS/Cu1.8S nanocomposites anodes exhibited a high initial discharge capacity (1555 mAh g −1 at 100 mA g −1 and 1130 mAh g −1 at 267 mA g −1 ), excellent cycle stability at high current density (average 425 mAh g −1 at 267 mA g −1 during 1000 cycles), and remarkable reversible rate performance, which could be attributed to the nano-size of the CuS/Cu1.8S nanocomposites and the assistance of Cu1.8S. Though the more suitable ratio of CuS to Cu1.8S needs to be further investigated, the high electrochemical performance of the CuS/Cu1.8S nanocomposites indicated that the CuxS nanomaterials will be a potential lithium-ion battery anode.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1: Figure S1: The Energy Dispersive Spectrometer of CuS/Cu1.8S nanocomposites, Figure S2: The electrochemical impedance spectroscopy after 1000 cycles with a frequency range of 10 −2 Hz-10 5 Hz, Table S1: The concentration of S and Cu element of CuS/Cu1.8S nanocomposites, Table S2: The comparisons of Rs, Rcf, and Rct before cycling and after 1000 cycles.

Conclusions
In this work, CuS/Cu 1.8 S nanocomposites were firstly prepared simultaneously by the one-pot Synthesis method at a relatively high reaction temperature 200 • C. The CuS/Cu 1.8 S nanocomposites anodes exhibited a high initial discharge capacity (1555 mAh g −1 at 100 mA g −1 and 1130 mAh g −1 at 267 mA g −1 ), excellent cycle stability at high current density (average 425 mAh g −1 at 267 mA g −1 during 1000 cycles), and remarkable reversible rate performance, which could be attributed to the nano-size of the CuS/Cu 1.8 S nanocomposites and the assistance of Cu 1.8 S. Though the more suitable ratio of CuS to Cu 1.8 S needs to be further investigated, the high electrochemical performance of the CuS/Cu 1.8 S nanocomposites indicated that the Cu x S nanomaterials will be a potential lithium-ion battery anode.