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Template-Free Electrochemical Preparation of Hexagonal CuSn Prism-Structural Electrode for Lithium-Ion Batteries
A hexagonal prism CuSn alloy was prepared at room temperature from 1-ethyl-3-methylimidazolium dicyanamide ([Emim][DCA]) by the direct template-free electrodeposition method with different concentrations of Cu(I) and Sn(II) at a low current density of 0.04 A dm−2. Moreover, the electrodeposition time was also investigated, and the results indicated that the composition of the CuSn alloy became complex and the structure turned unstable with expanding time. The cycling performance of the hexagonal prism-structural CuSn electrode was investigated, with the first discharge capacity of 345 mAh g−1 and a discharge capacity of about 210 mAh g−1 after 10 cycles.
Tin is well investigated as an anode material for lithium-ion batteries (LIB) owing to its high storage capacity (theoretical capacity is 993.3 mAh g−1) [1, 2]. However, during the charge and discharge process, Li intercalation and deintercalation causes the Sn electrode to undergo massive volume expansion and contraction due to the generation of LixSn alloy, which causes mechanical disintegration that leads to a poor cycle performance .
To stabilize the morphology of Sn anode materials, many efforts have been made to minimize the mechanical stress in the electrodes that cause the volume change. The results of investigations indicate that the Sn alloy [4–6] can effectively buffer the volume expansion during the charge/discharge process, thus improving structural stability and anode material cycle performance. Cu-Sn alloys are studied as anode materials for LIB due to their advantages, such as a stabilized structure, lower cost, environmental friendliness, and especially excellent conductivity. Moreover, the CuSn alloy often is the most commonly formed using template methods by chemical vapor deposition , sol-gel processing , and electrodeposition . As known to all, electrodeposition is widely used by industrial production and lab experiments due to its simple process, easy operation, uniform sedimentation, and so on. Furthermore, ionic liquids usually have a wide electrochemical window and show ideal electrolytes for electrodeposition application; it can especially obtain different microstructural platings in ILs [10, 11]. Sun et al. have reported a method that allowed electrochemical growth of metal alloys from ILs, obtaining a nanotube alloy  and a nanowire alloy .
Herein, we utilized the direct template-free electrodeposition method to synthesize CuSn from 1-ethyl-3-methylimidazolium dicyanamide ([Emim][DCA]). A prism structure of Cu-Sn was obtained, and it could provide a high surface area and a large number of active sites for charge transfer. Hence, this structure can alleviate the large volume change that induces mechanical disintegration during the charge/discharge process. Furthermore, the cycle performance of LIB, which has a hexagonal Cu-Sn prism as the anode, was also investigated.
The deposition electrolytes were prepared by the dissolution of various amounts of CuCl and SnCl2⋅2H2O (≥99.99%, Sigma-Aldrich) into [Emim][DCA] in the glove box. The electrochemical experiments were tested in a three-electrode system using a CHI660E electrochemical workstation. A Pt wire (Φ0.5 mm) was used as the counter electrode and a Pt foil immersed in deposition electrolytes was used as the reference. A Pt wire (Φ0.38 mm) was used as the working electrode for CV measurements, and a Cu foil (2 cm × 2 cm) was used as a substrate for electrodeposition.
The cell with the CuSn alloy as anode and lithium metal as cathode was assembled on a glovebox filled with argon (H2O, O2 < 0.5 ppm). The Celgard 2400 was the separator and the electrolyte was 1 M LiPF6 PC/DEC (v/v = 1 : 1). The cycling performance of the cell was cycled at a current density of C/10 at room temperature.
3. Results and Discussion
Figure 1(a) displays the cathodic polarization curves of [Emim][DCA] containing 0.05 M Sn(II), 0.1 M Cu(I) + 0.05 M Sn(II), and 0.1 M Cu(I). The curves have a rising current attributed to the electrodeposition of Sn (−0.66 V versus Pt), Cu-Sn (−0.75 V versus Pt), and Cu (−0.82 V versus Pt). This indicates that the addition of Sn(II) is beneficial to Cu electrodeposition with a Cu and Sn synergy codeposition process. Figure 1(b) shows cyclic voltammogram curves and it reveals that the Sn(II)/Sn couple  occur at a potential far more positive than the Cu(I)/Cu couple . Intriguingly, the two redox processes merge together and the potentials are shifted toward each other when the solution contains Cu(I) and Sn(II), suggesting that there is a strong interaction between Cu and Sn. In the CV, the reduction peak at −1.0 V and stripping peak at −0.47 V (vs. Pt) can correspond to the codeposition and strip of the Cu-Sn alloy, which is evidenced by the XRD spectra displayed in Figure 2. The results of the CV agree with the cathodic polarization.
Figure 2 shows SEM images for a typical Cu substrate electrodeposited with Cu-Sn in different concentrations of Cu(I) and Sn(II) at a current density of 0.04 A cm−2 for 1 h. Dendrite-like structures are obtained in the electrolyte with 0.04 M Cu(I) and 0.02 M Sn(II) (Figure 2(a)); with the increase in concentration, the Cu substrate is covered by a large quantity of outward growing Cu-Sn hexagonal prisms with diameters ranging from around 5 μm to 10 μm (Figure 2(b)). As the concentration continues to increase, uniform Cu-Sn hexagonal prisms are formed and the length of the prisms is about 20 μm (Figures 2(c) and 2(d)). The different sizes of the prisms may result from the initial nucleation time which is related to the concentration of Cu(I) and Sn(II). Higher concentration results from nucleation more easily produced and earlier nucleation formed larger prisms because there is a longer time period for the prism to grow .
The crystal structure of the as-deposited Cu-Sn prism at different deposition times was tested and shown in Figure 3. As shown in SEM images (Figures 3(a)–3(c)), the diameter of the Cu-Sn prism increases with the increase in deposition time. The tubular hexagonal Cu-Sn with an inner diameter of around 10–20 μm could be observed, but its structure is nonuniform, and its parts have some drawbacks (Figure 3(c)).
The diffraction patterns can be indexed to those of the Cu10Sn3, Cu3Sn, and Cu dominant peaks that resulted from the substrate (Figure 3(d)). As can be seen in Figure 3(d), there are other peaks (Cu5.6Sn and Cu4Sn) when the electrodeposition time is extended to 1.5 h. Moreover, the structure of CuSn is inhomogeneous and unsteady (Figure 3(c)). Furthermore, the ratio of the Cu-Sn alloy is examined with energy dispersive X-ray analysis (EDS), and the results reveal only Cu and Sn elements in the substrate with a 3.5 : 1 ratio of Cu/Sn, corresponding to the stoichiometry of Cu10Sn3. The formation of hexagonal Cu-Sn is unique and the growth mechanism is not very clear. Both the natural environment and the property of Cu and Sn metals provided by the ionic liquid may have an important role in hexagonal prism growth.
Figure 4 displays the cycling performance and coulombic efficiency of the Cu-Sn cell at a current density of C/10 at room temperature. The first discharge platform of the Cu-Sn cell is about 0.35 V and the first charge platform is 0.5 V (inset in Figure 4). The initial charge and discharge capacities are about 250 mAh g−1 and 345 mAh g−1, respectively. The initial cycle irreversible loss in capacities and lower coulombic efficiency can be seen in Figure 4. It may be attributed to the SEI film formed in the electrode surface and this is an irreversible process [15–17]. After 10 cycles, the discharge capacity stays at 210 mAh g−1 and the coulombic efficiency is about 100%. The capacity is lower than the theoretical capacity of Sn due to the Cu-Sn prism still undergoing volume expansion, but the hexagonal prism with Cu-Sn as the anode of LIB has an acceptable cycling performance [1, 18, 19]. Decreasing the size of the Cu-Sn prism or changing the condition during the preparation of nanostructural Cu-Sn probably could improve its capacity and cycle performance [20, 21].
The electrocrystallization of the hexagonal Cu-Sn prism, which is difficult to achieve in an aqueous solution, was obtained for the first time in a [Emim][DCA] ionic liquid at room temperature. Using the hexagonal Cu-Sn prism as the anode for LIB shows that the first discharge capacity is 345 mAh g−1 and the discharge capacity stays at 210 mAh g−1 after 10 cycles. The cell performance should be improved further; therefore, an in-depth study of this approach will provide a more precise control of the prism size for a synthesis of the nanostructure of the CuSn alloy for LIB.
(1) The cathodic polarization data used to support the findings of this study are included within the article. (2) The cyclic voltammogram data used to support the findings of this study are included within the article. (3) The XRD data used to support the findings of this study are included within the article. (4) The SEM images used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This project was supported by the National Natural Science Foundation of China (Grant no. 21276057).
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