Facile fabrication of CuS microflower as a highly durable sodium-ion battery anode

Cuihua An, Yang Ni, Zhifeng Wang, Xudong Li and Xizheng Liu*
Tianjin Key Laboratory of Advanced Functional Porous Materials, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, PR China. E-mail: xzliu@tjut.edu.cn

Received 7th February 2018 , Accepted 9th March 2018

First published on 12th March 2018


Sodium ion batteries (SIBs) have been considered as the promising substitutes for lithium ion batteries (LIBs) due to abundant resources of sodium. Metal sulfides have been demonstrated as prospective anode materials for SIBs based on a conversion mechanism. However, insufficient ionic transport and low conductivity in discharge electrodes prohibit their practical application. Herein, novel CuS microflowers were prepared by a facile dealloying method and applied as anode for SIBs. The microflowers are composed of nanosheets, which can provide increased Na+ diffusion admittance and more inter-space volume to accommodate volumetric change. When applied as anode in SIBs, the CuS microflowers-based anode delivered high discharge capacity (325.6 mA h g−1 at 0.1 A g−1) and excellent rate performance. The anode also displayed ultra-stable cycle performance and almost no capacity decay even after 5000 cycles (at a current density of 5 A g−1). Ex situ XRD was carried out to disclose the sodium ion storage mechanism. The CuS microflowers-based anode first experienced intercalation and then conversion mechanism with successive sodiation processes, while the reverse was observed for the charging process. The superior electrochemical performance is associated with the nano-micro structure and the controlled reaction mechanism. The current study states briefly that the simple and efficient dealloying method will provide more choices for fabricating anodes for SIBs.


Introduction

Sodium ion batteries (SIBs) have been considered a candidate for replacing lithium ion batteries (LIBs) in power stations due to the substantial deposits and cheapness of sodium resources.1–7 They possess a similar guest ion storage mechanism as LIBs and hence, most of the electrode materials for LIBs can be used for SIBs. However, the most popular graphite anode for LIBs does not seem work in SIBs due to severe sodium dendrite formation during discharging.8–11 Therefore, the development of a stable, low-cost and high performance anode for SIBs is one of the major obstacles for its practical application. Nevertheless, there are some shortcomings restricting usage such as insufficient cycle stability, lower discharge capacity, as well as high production cost.12–14 Aiming at resolving the above challenges, the design and fabrication of nano-micro structured materials or composites have been adopted and regarded as the most effective means in the past few years.15–18 It is urgent to seek appropriate electrode materials, which is the impetus for promoting SIBs.

Presently, multifarious metal oxides, sulfides and their hybrids with conductive additions have been studied and reported as anode materials for SIBs.19–23 Specifically, metal sulfides, which possess high electrical conductivity and theoretical capacity, have aroused the interest of scientists. In addition, transitional metal sulfides with distinctive microstructures have been the most applied materials, for instance, FeS2 nanospheres, NiS spheres, CoS2 micro/nano-structure, MoS2 nanoflowers, and few-layered SnS2.24–30 Among them, Cu2S has a series of advantages of high security, cost-effectiveness, superior conductivity and high theoretical capacity when used as an electrode material.31–33 The reported Cu2S electrode obtained via ball-milling delivered a discharge capacity of 294 mA h g−1 in the first cycle, which decreased to 261 mA h g−1 at 20 cycles when the current density was 50 mA g−1.34

To date, various CuS microstructures, such as one-dimensional (1D) rods and tubes, 2D plates and sheets, and 3D spheres and flowers, have been prepared and applied as electrodes.35–40 Kalimuldina et al. reported CuS coated on carbon fiber paper for LIBs by spray pyrolysis at relatively high temperature.41 Li et al. synthesized CuS nanosheets via a microwave hydrothermal method and adopted thioacetamide as the sulfur source.42 In the existing processes for producing nano-micro structured CuS materials, thioacetamide, thiourea and sodium sulphide usually act as sulfur sources, due to which the foul-smelling hydrogen sulfide is generated, which would contaminate the environment. However, mass production can hardly be realized based on these methods. Therefore, the investigation of a new CuS fabrication method can be enlarged to meet commercial utilization.

Herein, a simple chemical dealloying method at room temperature was introduced to prepare nano-micro structured CuS materials. The morphologies of CuS materials were tuned by optimizing the dealloying time. Their sodium ions storage properties were studied and the phase evolution accompanied with discharge/charge was revealed by ex situ XRD.

Experimental section

Synthesis of CuS material

The CuS fabrication process is shown in Scheme 1. Ti–Cu alloys were prepared by melting of high purity Ti and Cu in the atomic ratio of 60[thin space (1/6-em)]:[thin space (1/6-em)]40. Then, the Ti60Cu40 alloy-strips with 50 μm thickness were fabricated through a re-melting and spinning procedure. Finally, 0.1 g Ti60Cu40 alloy strips were added to 50 mL water with constant stirring. Then, 12 M H2SO4 aqueous solution (50 mL) was added to the above solution dropwise. The above solution was heated to 90 °C for certain durations (24 h, 36 h or 48 h). The as-obtained samples were washed with water and ethanol, and dried at 60 °C. The corresponding products were denoted S24, S36 and S48. The productivity of CuS by this dealloying method can be calculated by the following equation:
image file: c8qi00117k-t1.tif
in which mactual is the actual mass of CuS after the sulfidation reaction, mtheoretical is the theoretical mass of CuS according to the mass of Cu in the Ti60Cu40 alloy. The actual mass of the CuS sample is 0.0191 g. The theoretical CuS mass is 0.071 g based on 0.1 g Ti60Cu40 alloy. Therefore, the productivity of the as-synthesized CuS is 26.9%. After the sulfidation reaction, the colour of the solution was blue-green, which indicates the presence of Cu2+ ions. Moreover, copper reacts with 12 M H2SO4 solution to produce soluble CuSO4, except for CuS, which may be the cause of the low yield.

image file: c8qi00117k-s1.tif
Scheme 1 Illustration of the preparation process for the different structure copper sulfide samples.

Characterization

The material structure was determined by X-ray diffraction (XRD, Rigaku Ultima IV, Cu-Kα radiation). The microscopic structures and morphologies were characterized via field emission scanning electron microscopy (FESEM, FEI Verios 460L), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) on a FEI Technai G2 Spirit TWIN. X-ray photoelectron spectrometry (XPS, ESCALAB250Xi, Thermo Scientific) was also conducted to study the valence states information. The porous features and specific surface areas of the as-obtained CuS materials were estimated by nitrogen adsorption/desorption measurements (NOVA 2200e).

Electrochemical measurements

The obtained CuS materials were used as anodes for SIB to evaluate the electrochemical performance. To fabricate the working electrode, the obtained CuS material, acetylene black and methylcellulose Na were mixed in water in the 70[thin space (1/6-em)]:[thin space (1/6-em)]20[thin space (1/6-em)]:[thin space (1/6-em)]10 ratio to form a slurry. The slurry was deposited on Cu foil and then dried in an oven overnight. The mass loading was about 1 mg cm−2. The electrochemical performances were evaluated using CR2032-type coin cells, which were packaged in an argon-filled glove box. Sodium metal was used as the counter and reference electrodes. Glass fiber (Whatman GF/A) was used as the separator and 0.5 mol L−1 NaClO4 in diethylene glycol dimethyl ether (DME) was employed as the electrolyte. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were characterized on a CHI 760E electrochemical workstation. Galvanostatic discharge/charge cycles were conducted on a LAND battery test instrument (CT2001A).

Results and discussion

The XRD pattern of the Ti60Cu40 alloy-strip is depicted in Fig. S1. There is only a broad peak in the range 35–50°, which demonstrates an amorphous alloy similar to that in previous literature.43–46 The CuS fabrication process is shown in Scheme 1. After dealloying, the diffraction peaks changed as shown in Fig. 1a. Interestingly, in our experiment, XRD patterns of the products were almost the same after 24, 36 and 48 h. In addition, the characteristic peak of the Ti60Cu40 amorphous alloy disappeared and was replaced by six sharp peaks associated with hexagonal copper sulfide (JCPDS no. 01-1281). No other diffraction peaks were present, demonstrating complete transformation from Ti60Cu40 amorphous alloy to the CuS phase. The specific reaction process can be depicted as mentioned below:
 
2Ti + 6H2SO4 → Ti2(SO4)3 + 3SO2 + 6H2O (1)
 
5Cu + 6H2SO4 → 4CuSO4 + CuS + SO2 + 6H2O (2)

image file: c8qi00117k-f1.tif
Fig. 1 XRD patterns (a). XPS spectra of Cu 2p (b) and for S 2p (c) in S24, S36 and S48. SEM images of S24 (d), S36 (e) and S48 (f). TEM image (g), HRTEM (h) and SAED pattern (i) of S48 material.

To further identify the composition and valence of the as-obtained CuS samples, XPS measurements were conducted (Fig. 1b and c). The sharpnesses and locations of the peaks were almost identical, but with different peak intensities, suggesting that they had the same composition and valence states (Fig. 1b and c, Fig. S2). Specifically for S48, the Cu 2p spectra possessed two main peaks and two satellite peaks. The fitted peaks located at 932.4 and 934.5 eV and the deconvoluted peaks at 952.2 and 954.3 eV could be attributed to Cu 2p3/2 and Cu 2p1/2 of CuS, respectively.47,48 Moreover, the XPS spectra for S 2p could be fitted to three peaks centered at 161.5, 162.3 and 163.5 eV (Fig. 1c). The first two abovementioned peaks could be assigned to the Cu–S bond and the last peak to the S–S bond.49,50 Therefore, according to the XRD and XPS results, copper sulfide samples were triumphantly prepared and obtained.

The morphologies and microstructures of the copper sulfide samples were recorded and confirmed by SEM and TEM as depicted in Fig. 1d–i. The three samples S24, S36 and S48 have different structures from flakelet and slice to nano-flowers, which were built by numerous slices. When the reaction time was only 24 hours, the obtained S24 sample composed of countless flakelets with thickness of around 15 nm (Fig. 1d). On increasing the reaction time to 36 hours, the S36 still retained the sheet morphology, but the thickness of the slice doubled, indicating the thickness increment during this period. On further prolonging the reaction time, the micro-structure of the S48 sample changed drastically and completely differed from the above two materials. In addition, the S48 material presented nano-flowers built by numerous nanosheets. The thickness of a constituent slice was about 30 nm, which was nearly the same as that in S36 (Fig. S3). The above result demonstrated that there were two main procedures, the growth of flakelets and the self-assembly of slices. Subsequently, the structure of the S48 sample was identified by TEM as presented in Fig. 1g, through which it was clarified that S48 nanoflowers were composed of nanosheets and the diameter of the nanoflower was about 400 nm, which agreed with the SEM results. The HR-TEM image (Fig. 1h) indicated that the structure of the S48 nanoflowers exhibited an interplanar distance of 0.322 nm indexed to the (101) plane of CuS. In addition, the SAED pattern of the S48 sample exhibited distinct rings, certifying the polycrystallinity.

Specific surface areas and pore sizes were characterized by N2 adsorption–desorption measurements (Fig. S4). All three materials S24, S36 and S48 depicted type-IV isotherms, suggesting their porous features. In addition, the specific surface area increased from 48.2 to 67.9 m2 g−1 with reaction time. Nevertheless, the pore size distribution of these three materials decreased with reaction time. Among them, S48 possessed the largest specific surface area (67.9 m2 g−1) and the smallest pore size (12.1 nm), which could offer the highest Na+ diffusion admittance, better electrode/electrolyte interface contact and highest inter-space volume to accommodate volumetric change.51,52

The comparison of the rate performances of S24, S36 and S48 is displayed in Fig. 2a. Specifically, the S48 electrode appeared to have the highest discharge capacities. As the current density increased from 0.1 to 1 A g−1, the S48 electrode possessed discharge capacities of 329.3, 265.8, 228.6 and 195.7 mA h g−1. As the current density increased to 2 A g−1, the capacity of the S48 electrode remained as high as 164.6 mA h g−1. Subsequently, by reversing the current density to 0.1 A g−1, a nearly similar discharge capacity of 323 mA h g−1 was obtained, suggesting excellent capacity retention of the S48 electrode. On the contrary, the S24 and S36 electrodes delivered reversible capacities of 146 and 128.3 mA h g−1 at 2 A g−1, respectively. These values regained to 298 and 277.3 mA h g−1 at 0.1 A g−1, which were less than that of the S48 electrode.


image file: c8qi00117k-f2.tif
Fig. 2 Rate performances of S24, S36 and S48 electrodes (a), CV curves at 0.1 mV s−1 (b), charge–discharge curves at 0.1 A g−1 (c), cycle performance of the S48 electrode at 5 A g−1 (d).

Cyclic voltammetry (CV) was conducted to further investigate the electrochemical properties of the S48 electrode (Fig. 2b). There were two peaks (1.52 V and 0.87 V) in the first cathodic scan and three peaks (1.39 V, 1.95 V and 2.18 V) in the corresponding anodic scan. During the second and third cycles, the peak located at 0.87 V disappeared due to the irreversible reaction, namely, the SEI layer formation during the first cycle.53,54 Another peak existed in both the cathodic and anodic scans. The above results indicated that Na+ intercalation/deintercalation of the S48 electrode was not merely an ordinary one-step conversion reaction. The particular reaction mechanism will be discussed below. The shape and peak positions in the second and third cycles were nearly unchanged, demonstrating good stability of the S48 electrode.

The galvanostatic charge/discharge (GCD) curves of the S48 electrode in the first three cycles at 0.1 A g−1 are depicted in Fig. 2c. Two distinct discharge and charge plateaus were discovered, which were in agreement with the CV results. The primal discharge capacity reached 483 mA h g−1, which was 86% of the theoretical capacity (561 mA h g−1). The initial colulombic efficiency (CE) was about 90.9%, which was attributed to the ether-based electrolyte.29,50,55 The discharge capacities of the second and third cycles were 329.3 and 324.6 mA h g−1, respectively. In addition, subsequent CE was nearly 98%, indicating outstanding reversibility of the S48 electrode. Moreover, the GCD curves at various current densities are shown in Fig. S5. Surprisingly, the forms and locations of the plateaus in the various GCD curves for the S48 electrode looked almost the same, implying good rate performance and low polarization. The discharge capacities of the S48 anode at various current densities from 0.1 to 2 A g−1 were 325.6, 265.8, 228.7, 195.7 and 164.6 mA h g−1. The cycling stability of the S48 anode at relatively high current density of 5 A g−1 is exhibited in Fig. 2d. The S48 anode delivered the highest discharge capacity of 132.6 mA h g−1, which remained stable until 5000 cycles. This also indicated that the capacity retention of the S48 anode was nearly 100% after 5000 cycles, indicating its prominent cycling stability. Due to the mild activation procedure of the first few cycles, CE of the S48 anode increased from 94.7% to 95.2%. Moreover, the S48 electrode exhibited charge and discharge capacities of 164 and 154.4 mA h g−1 after the first and 3000 cycles, respectively, at 2 A g−1, which was almost undiminished during the next 3000 cycles (Fig. S6). The abovementioned consequences declared that the S48 anode appeared to have the enhanced rate performance and cycling stability, which were associated with the neoteric 3D flower-like structure. This structure could provide more Na+ diffusion paths and more interspace to tolerate volumetric change during the charge/discharge procedure.

The CV curves and GCD curves of the S24 and S36 electrodes at various current densities are displayed in Fig. S7. Interestingly, we see that the GCD curves show almost identical shapes, peak positions and plateau locations, but reduced discharge capacity and subdued rate performances. This might be because the insecure structures and smaller specific surface areas of the two electrodes could not resist the enormous volume expansion during sodiation/desodiation. The variation in the micro-structures of these three electrodes will be discussed below in detail.

The ex situ XRD measurements of the S48 anode were conducted to figure out the reaction mechanism (Fig. 3). To avoid the background peak of the Cu current collector, Al foil was adopted as the current collector only in the ex situ XRD test. Therefore, there were three intensive peaks in all the XRD patterns that belonged to the Al foil substrate. Moreover, the reflection peak of the intermediate NaCu2S2 was discovered under incomplete discharge/charge conditions (Fig. 3a and c). Diffraction maxima of the CuS also existed, manifesting that the sodiation/desodiation reactions involved the formation of the NaCu2S2 intermediate. In the fully discharged status, elemental copper and Na2S with weak intensity were detected. Similarly, the peaks of the original CuS appeared in the fully charged stage. To further prove the aforementioned ex situ XRD results, SAED patterns of the various charged/discharged products are shown in Fig. 4. As shown in Fig. 4a, while discharging to 1.4 V, the very weak diffraction rings were attributed to NaCu2S2, in which interplanar spacings of 0.30 and 0.24 nm could be ascribed to the (101) and (102) facets, respectively. Sequentially discharging to 0.3 V (Fig. 4b), two reflection rings were discovered and classified as the (220) facet of Na2S (0.23 nm) and the (111) facet of Cu (0.21 nm). Then, on charging to 2.0 V (Fig. 4c), only one distinct ring appeared and was categorized as the (110) facet of NaCu2S2 (0.23 nm). On further charging to 2.2 V (Fig. 4d), the diffraction rings of polycrystalline CuS appeared once more and the spacings of 0.19 and 0.33 nm were regarded as the (110) and (101) facets, respectively. Overall, the sodiation/desodiation procedures could be divided into two steps: insertion/extraction Na+ into the S48 anode to form the intermediate NaCu2S2, and conversion to Na2S and Cu or CuS. Na+ successfully inserted in the crystal structure of CuS without structural damage (Fig. 4e and f). Therefore, the reaction process can be summarized as follows:

2CuS + Na+ + e ↔ NaCu2S2

NaCu2S2 + 3Na+ + 3e ↔ 2Na2S + 2Cu


image file: c8qi00117k-f3.tif
Fig. 3 Ex situ XRD patterns of S48 in various discharged or charged states.

image file: c8qi00117k-f4.tif
Fig. 4 SAED patterns of the S48 electrode after discharging to 1.4 V (a), 0.3 V (b), and charging to 2.0 V (c) and 2.2 V (d). Crystal structure of CuS (e) and intermediate NaCu2S2 (f).

The micro-structure variation of the S48 electrode profoundly affected its electrochemical properties. Hence, the morphologies of the S48 electrode after 300 and 500 cycles were characterized by SEM (Fig. 5). As compared to the pristine structure, the 3D flower-like structure was destroyed and collapsed to a certain degree (Fig. 5a and c). As shown in Fig. 5b and d (zoom in), nanosheet building blocks still existed after 300 cycles, while being replaced by some amount of bulk material after 500 cycles. The same phenomenon was observed for the S48 electrode after 5000 cycles at 5 A g−1 (Fig. S8). These observations might be the reasons for capacity fading in the long term cycling.


image file: c8qi00117k-f5.tif
Fig. 5 SEM images of the S48 electrode after 300 cycles (a, b) and 500 cycles (c, d) at 1 A g−1.

EIS spectra of the S24, S36 and S48 electrodes were recorded to further illustrate the causes for performance distinction (Fig. S9a). EIS spectra of the S48 electrode after various cycles are displayed to account for capacity attenuation (Fig. S9b). As listed in Table S1, Rct of fresh S48 was only 83.3 Ω, which was much less than those of the S24 (288.8 Ω) and S36 (220.1 Ω) electrodes. We know that Rct represents the charge transfer resistance at the electrode/electrolyte interface. The reduced Rct of S48 illustrated that the S48 electrode with nano-flower morphology provides more ionic/electronic transportation pathways and thus facilitates charge transfer at the electrode/electrolyte interface. With an increase in number of cycles, Rct of the S48 electrode increased from 83.3 to 210.9 Ω after the 80th cycle and then maintained a gradual increment (Table S2). This may be because of the accumulation of side products during long term cycling.

Conclusions

CuS was fabricated by dealloying Ti–Cu alloy and the morphology was optimized with dealloying time. The dealloying method simplifies the preparation process, reduces the total cost, and avoids the emission of the harmful gas H2S, which is usually generated in the conventional preparation process of transition metal sulfides. The 3D nano-flower CuS (S48) was obtained after dealloying for 48 h and possessed the largest specific surface area and smallest pore size. Adopted as an anode for SIBs, S48 delivered high discharge capacity (325.6 mA h g−1 at 0.1 A g−1), excellent cycling stability (almost 100% capacity retention after 5000 cycles) and outstanding rate performance. The ex situ XRD disclosed that a two-step process occurred involving insertion/extraction of Na+ and conversion to Na2S and Cu. In addition, there is an opportunity for improving the electrochemical properties of the CuS anode such as by using a carbon composite, the study of which is underway. The current dealloying and sulfuration method will be extended to the synthesis of other transition metal sulfides to provide more choices for designing anodes for Na-ion batteries with improved performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (51601127, 51671145), Tianjin Sci. & Tech. Program (17JCYBJC21500) and major projects of new materials of Tianjin city (16ZXCLGX00120).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qi00117k

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