Electrocodeposition of lithium and copper from room temperature ionic liquid 1-ethyl-3-methyllimidazolium bis(trifluoromethylsulfonyl)imide

Abstract

1-Ethyl-3-methyllimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][TFSI]) ionic liquid containing Li(I), and [EMIm][TFSI] with N-methyl-2-pyrrolidone (NMP, volume ratio 2 : 1) containing Cu(II) and mixtures of Li(I) and Cu(II) was studied using cyclic voltammetry at 298 K. NMP was used to increase the solubility of Cu(II) in the [EMIm][TFSI]. The underpotential deposition (UPD) of lithium was observed at about −2.0 V (vs. Pt). Electrodeposits of Li–Cu with different atom content of Li can be obtained by controlled-potential electrolysis. The XRD and XPS results show that the deposit consists of metallic lithium and copper. The Li–Cu alloy coatings obtained in this study were compact and adherent, and it was clearly found that there was Cu framework left after dissolving deposited Li in distilled water.

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1. Introduction

Room temperature ionic liquids (RTILs) have a number of attractive characteristics, including a wide electrochemical window, low vapor pressure, high thermal stability, good ionic conductivity and no hydrogen evolution.^1,2 Due to their wide electrochemical window, RTILs have proven to be good solvents for electrodeposition of metals, alloys and semiconductors, especially for the electrodeposition of reactive metals that cannot be deposited from aqueous solutions.^3–5

Of all the metals, reduction potential of Li is one of the lowest, and it has been reported that lithium layer could be obtained by electrodeposition from RTILs. Endres et al. prepared the macroporous aluminium electrodes by template assisted electrodeposition from ionic liquids [EMIm]Cl/AlCl₃ (40/60 mol.%) and then deposition and stripping of lithium on the macroporous electrode in [Py1,4][TFSA] were studied.⁶ Wibowo et al. investigated kinetic and thermodynamic parameters of the Li/Li⁺ couple in [C₄mpyrr][NTf₂] between 298–318 K.⁷ Experiments are conducted using both nickel and platinum microelectrodes.

Metallic Li can be used as an anode material for Li-ion batteries, but the dendrite formation of Li during charging process leads to irreversible capacity loss of Li-ion batteries.⁸ As the excellent stability of Cu in Li-ion batteries, Li–Cu alloy can be used as an appropriate anode material for Li-ion batteries since the Cu frame obtained by dissolving deposited Li may inhibit the dendrite formation of Li during charging process. In 1995, Lambri and Peñaloza prepared copper–lithium (Cu–Li) alloy coatings by electrodeposition using a sheet of high-purity copper as the cathode in a bath of fused salts of LiCl and KCl, separately.^9–11 The obtained Cu–Li alloy has 18 at.% (atom content) lithium content only. The codeposition of two metals that have such a large separation of the deposition potentials is hard to be realized in traditional solvent. In view of that Chen and his coworkers obtained Cu–Mn alloy from BMP-TFSI by electrodepositing, the codeposition of Li–Cu may be put into practice from RTILs.¹² Unfortunately, many metal salts have low solubility in RTILs with weakly coordinating anions like [BF₄]⁻, [PF₆]⁻, or [Tf₂N]⁻.^13–15 However, it was reported that the copper compounds hardly dissolved into the RTILs with [TFSI]⁻.^16,17 The method employed was anodic dissolution to introduce metal ions into RTILs.

There are a few studies about additives in depositions from RTILs. Sano et al. investigated the behavior of Li electrodeposition on nickel electrodes in RTILs, using in situ optical microscopy with/without vinylene carbonate (VC) as an organic additive.¹⁸ The result showed that the addition of VC in PP13[TFSA] could suppress the Li dendritic growth. Chen studied the electrochemical behavior of the Li⁺/Li couple at different electrodes in tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide ionic liquid mixed with a little propylene carbonate (PC) at 303 K.¹⁹ The addition of small amounts of PC enormously decreased the viscosity so that mass transfer becomes more efficient.

In the present study, to demonstrate the feasibility, Li–Cu alloy coatings with high lithium content were electrodeposited on copper substrates from 1-ethyl-3-methyllimidazolium ([EMIm][TFSI]) ionic liquid with N-methyl-2-pyrrolidone (NMP). Li–Cu alloy may be used as a new Li-ion battery anode material to improve its charge–discharge cycling properties, as Cu framework may prevent Li dendrite formation during charging. The voltammetric behaviors of Li(I), Cu(II), and mixtures of Li(I) and Cu(II) were studied. NMP was adopted as an additive in this study to increase the solubility of copper ions in [EMIm][TFSI]. The deposition potential and temperature effects on component of the deposits were also investigated.

2. Experimental

Ionic liquid [EMIm][TFSI] was purchased from Shanghai Chengjie Chemical Co. Ltd and dried under vacuum for 12 h at 393 K and stored in an argon filled glove box. Water content in the ionic liquid was determined to be less than 10 ppm with a coulometric Karl Fischer titrator (SFY-3000, Haifen Instment Ltd. Co., China). Anhydrous copper p-toluenesulphonate (Cu(p-oTs)₂) was prepared by direct reaction of Cu₂(OH)₂CO₃ and p-toluenesulfonic acid. LiTFSI was purchased from Shanghai Chengjie Chemical Co. Ltd. Both were stored under argon atmosphere, and before using, dried under vacuum.

The electrolyte containing Li(I) and Cu(II) was prepared by adding appropriate amount of Cu(p-oTs)₂ and LiTFSI into [EMIm][TFSI] with additive NMP in glove box filled with dry argon. The solution was stirred by magnetic stirrer for several minutes to ensure the complete dissolution of Cu(p-oTs)₂ and LiTFSI. Electrodepositions were carried out on Cu foil. The deposits were washed in NMP to remove the ionic liquid residue in the glove box.

Electrodeposition and electrochemical measurements were carried out in a system filled with dry argon. [EMIm][TFSI] ionic liquid was poured into a self-made glass electrolytic cell in the glove box. Electrodeposition of Li–Cu alloy from [EMIm][TFSI] with NMP and electrochemical measurements were performed in a home-made 3-electrode cell by Princeton Applied Research 2273 potentiostat/galvanostat (PAR 2273). A copper wire (0.38 mm diameter) served as the working electrode (WE), considering copper is common current collector in Li-ion battery. The counter electrode (CE) was a platinum plate and the quasireference electrode was a platinum wire (0.5 mm diameter). A piece of copper plate (10 × 10 mm² in area) was employed replacing the copper wire as the working electrode for the electrodeposition of the Cu, Li, or Li–Cu alloy coatings. During the electrochemical experiments, the temperature of the electrolyte was kept at room temperature (298 K). Surface morphologies of the deposits were characterized by field-emission scanning electron microscope (FE-SEM, Hitachi S4700). Deposits were dissolved into solution by nitric acid and analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES, Perkin Elmer, 5300DV) to determine the composition of the deposits. X-ray diffraction (XRD) measurement was carried out with a 2θ range of 10–90°, using Cu radiation, on a Philips X'Pert diffractometer. Measurement conditions were 40 kV and 40 mA. The electrodeposited samples were 10 × 10 mm² in area. X-ray photoelectron spectroscopy (XPS) analysis was taken by PHI 5700 ESCA System with monochromatic Al Kα radiation (1486.6 eV). All spectra were calibrated, so the binding energy of the C 1s peaks was 284.6 eV.

3. Results and discussion

In order to obtain the voltammetric behavior of Li ions in [EMIm][TFSI], different cycles of cyclic voltammogram of Li were studied, as shown in Fig. 1. The potential was initially scanned from open circuit potential towards −4.2 V (vs. Pt) and then back to 0.5 V. The reduction peak c₁ (about −2.0 V) in Fig. 1 represents the underpotential deposition (UPD) of lithium. A large “nucleation loop” appears in the range of potential from −2.7 V to −4.0 V, which means the growth of nuclei of lithium occurring at the copper electrode. The fact that the nucleation peak c₂ is larger than a₁ represents not only the deposition of lithium but also decomposition of [EMIm]⁺ as the decomposition potential of [EMIm]⁺ is around −2 V. It has observed that the addition of lithium salts to RTILs causes a significant enlargement of the cathodic branch of the electrochemical window since the cathodic limit of [EMIm][TFSI] is −2 V(vs. Pt).^9,20 It is possible that a solid electrolyte interphase (SEI) forms with a reaction between the deposited Li and the decomposed electrolyte.^21,22 SEI is Li⁺-conductive and prevents the reduction of cation of ionic liquid. No anodic peak of UPD can be found in the curve, it is thought the UPD of lithium is involved into the formation of SEI. Furthermore the UPD peak of lithium disappears in the second cycle, which may due to the incomplete stripping of deposited Li in the first cycle. In the second and third cycles, the deposition and stripping peak of lithium decreased sharply. It is suspected that the SEI formed perfectly in the 1st cycle. As a result, reduction of [EMIm]⁺ and deposition of Li⁺ were all hindered and SEI is Li⁺-ion conductive, so the deposition of lithium still can be shown by the stripping peaks.

Fig. 1 Different cycles of cyclic voltammograms recorded at the Cu electrode in [EMIm][TFSI] containing 1 mol L⁻¹ Li(I). Scan rate: 10 mV s⁻¹. Temperature: 298 K.

Due to the low solubility of copper compounds in [EMIm][TFSI], NMP was added to make the copper compounds dissolve into [EMIm][TFSI]. In order to find out the effects of additive on [EMIm][TFSI], the cyclic voltammograms of [EMIm][TFSI] ionic liquid and [EMIm][TFSI] with NMP (volume ratio 2 : 1) were studied (shown in Fig. 2). The [EMIm][TFSI] ionic liquid exhibits an electrochemical window of about 4.2 V (vs. Pt). No apparent oxidation peak and reduction peak of impurities were observed in the CV of pure ionic liquid. It is found that an anodic peak appears at 1.2 V in the CV of [EMIm][TFSI] with NMP, and there is no obvious difference in the cathodic potential branch. As a result, NMP will reduce the electrochemical window of [EMIm][TFSI] to about 3 V. Nevertheless, NMP just decreases the anodic potential limit of [EMIm][TFSI]. There was no clear effect on cathodic potential branch. Therefore NMP will not affect the electrodeposition process of Li–Cu.

Fig. 2 The cyclic voltammograms of a) [EMIm][TFSI] ionic liquid and b) [EMIm][TFSI] with NMP (volume ratio 2 : 1) recorded at Pt working electrode. Scan rate: 10 mV s⁻¹.

The voltammetric behavior of the Cu(II) was studied in [EMIm][TFSI] with NMP (volume ratio 2 : 1) at the Cu electrode, as shown in Fig. 3, from −1.0 V in the anodic direction towards the switching potential and then back to −1.0 V. The voltammogram reveals two reduction couples, c₁ and c₂ represent the processes of Cu(II) reducing to Cu(I) and Cu(I) reducing to Cu(0), respectively. It is found that the area of peak c₁ is larger than that of peak c₂ in Fig. 3. The reason may be that Cu(I) derived from reduction of Cu(II), and its concentration was much lower than Cu(II). The stripping peak at 0.02 V is due to the oxidation of deposited Cu. The a₂ wave is due to decomposition of NMP, because the anodic limit of ionic liquid is more positive, as shown in Fig. 2.

Fig. 3 Cyclic voltammograms recorded at the Cu electrode in [EMIm][TFSI] with NMP (volume ratio 2 : 1) containing 0.066 mol L⁻¹ Cu(II). Scan rate: 10 mV s⁻¹. Temperature: 298 K.

The cyclic voltammograms recorded at the Cu electrode in [EMIm][TFSI] with NMP (volume ratio 2 : 1) containing a mixture of Li(I) and Cu(II) with a concentration ratio of [Li(I)]/[Cu(II)] = 0.77 M/0.066 M are shown in Fig. 4. Two reduction peaks could be observed clearly. In contrast with the reduction process in Fig. 3, peak c₁ represents the reduction of copper. By carefully comparing Fig. 4 with Fig. 1, it can be found that Li(I) in the mixture was reduced to Li(0) at more positive potential, as indicated by the positive shift in the reduction peak of Li(I). This behavior implies that Li(I) and Cu(II) have certain interactions, resulting in a smaller separation of the reduction potentials. This behavior is beneficial to Li–Cu codeposition. While there is just one stripping peak a₁ in Fig. 4, it is indicated that Li and Cu formed alloy and stripped together. A similar situation was reported by Heish in the study of codeposition of Cu and Sn.²³

Fig. 4 Cyclic voltammograms recorded at the Cu electrode in [EMIm][TFSI] with NMP (volume ratio 2 : 1) containing 0.066 mol L⁻¹ Cu(II) and 0.77 mol L⁻¹ Li(I). Scan rate: 10 mV s⁻¹. Temperature: 298 K.

The electrodeposition of Li–Cu alloy coatings was investigated by bulk controlled-potential electrolysis on copper plates from [EMIm][TFSI] with NMP (volume ratio 2 : 1). Dense and fine coatings of Li–Cu coatings were obtained. The deposits obtained at different potentials were studied by ICP, and the results were shown in Table 1. We found that the atom content of Li in the deposits increased obviously with the increasing of applied potential. This behavior is reasonable due to the different reduction potentials of Cu(II) and Li(I). At the potentials which the deposition of Li occurs, the reduction of Cu(II) is completely under the control of diffusion. The reduction rate of Li(I), however, is controlled by the applied potentials. Therefore, the content of Li in the Li–Cu deposits increased with more negative applied potentials. Table 1 shows that Li–Cu coatings with different atom content of Li can be obtained by regulating the applied potential.

Table 1 Atom content of Li in Li–Cu coatings obtained at different potential

Potential/V	−2.5	−3.0	−3.5
Li content at.%	29	37	56

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The effect of temperature on the atom content of Li in the Li–Cu deposit was also investigated, as shown in Table 2. As can be seen, the content of Li in Li–Cu alloy coatings depends on the temperature. The atomic ratio of Li decreased as the temperature increased. The reason is that reduction of Cu(II) is under the control of diffusion when the deposition of alloy occurs. When the temperature was increased, the concentration polarization of Cu(II) decreased as Cu(II) in the electrolyte diffused faster towards cathodic electrode. So the deposition of Cu became easier, and its content in the Li–Cu deposits increased.

Table 2 Li content of the electrodeposit obtained at different temperature

T/K	293	303	313
Li content at.%	51	46	39

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The obtained 56 at.% Li–Cu coating was chosen here due to its high atom content of Li. The sample was studied by XRD in order to analyze the phases of Li–Cu alloy deposits. The diffraction pattern of the sample that electrodeposited on nickel foil is shown in Fig. 5. However, the diffraction peaks of copper substrate would overlap the diffraction peaks of copper in Li–Cu alloy, so the nickel was used to replace the copper as substrate. In Fig. 5, there are Ni(111), Ni(200) and Ni(220) diffraction peaks that related to the nickel substrate at approximately 2θ = 44.5°, 52.1° and 74.13°, as the Li–Cu alloy coating is so thin to be penetrated during the test by X-ray. LiOH(101) and LiOH(110) are clearly seen at 2θ = 32.5° and 35.6°. There are also Cu(111), Cu(200) and Cu(220) appearing at 2θ = 43.2°, 53.4° and 74.2°. The LiOH originated from production of Li reacting with water in the air during test process, as the test was not in a vacuum. The result indicates that lithium and copper in Li–Cu coating exist as independent metals.

Fig. 5 XRD pattern of Li–Cu coatings electrodeposited on nickel foil obtained from a solution of 0.066 mol L⁻¹ Cu(II) and 0.77 mol L⁻¹ Li(I) in [EMIm][TFSI] with NMP (volume ratio 2 : 1)) at −3.5 V (vs. Pt).

Fig. 6(a) and (b) show detailed XPS spectra of the Li 1s, the Cu 2p1/2 and the Cu 2p3/2 peaks, respectively. The main C 1s peak was located at 285.54 eV. A rough compensation for the charging effects was carried out by shifting the energy scale so that the main C 1s peak obtained a binding energy of 284.6 eV. Three peaks were used in order to obtain a satisfactory fit for the Li 1s spectra. The lower energy peak a1 at 54.6 eV is believed to originate from metallic lithium. The peak a2 at 55.6 eV is ascribed to lithium oxide and the peak a3 at 54.9 eV is associated with LiOH. The Cu 2p3/2 spectrum was also fitted with three peaks, Cu₂O at 932.4 eV, Cu at 932.6 eV and CuO at 933.8 eV. The Cu 2p1/2 peak appearing demonstrates the existence of the copper oxide. It can be fitted by two peaks. One is at 952.5 eV representing Cu₂O, the other is at 953.7 eV representing CuO. The XPS results show that not only metallic lithium and copper but also their oxides exist in the experiment. The oxides may be introduced when the sample is exposed to air before being placed into the sample room. Therefore it is supposed that the deposit consists of metallic lithium and copper. It corresponds to the result of XRD.

Fig. 6 XPS spectra showing Li 1s (a), Cu 2p3/2 and Cu 2p1/2 (b) photoelectron peaks, respectively. The spectra were recorded for Li–Cu coatings electrodeposited on Cu foil obtained from a solution of 0.066 mol L⁻¹ Cu(II) and 0.77 mol L⁻¹ Li(I) in [EMIm][TFSI] with NMP (volume ratio 2 : 1)) at −3.5 V (vs. Pt).

The morphologies of two selected samples were studied. One sample was the deposits obtained at −3.5 V (vs. Pt) and the other was obtained by immersing the former deposits into distilled water for 5 min. The surface morphologies of these samples are shown in Fig. 7. A comparison of Fig. 7a and Fig. 7b reveals that after immersing the deposit into distilled water for 5 min, some pores appear on the deposit which are caused by the deposited lithium reacting with distilled water. It is indicated that lithium and copper could form Li–Cu alloy and kept their chemical properties. The structure conforms to our initial assumption that copper frame can be obtained after dissolving of deposited Li in Li–Cu deposits. We can expect that the obtained Cu frame may inhibit the dendrite formation of Li during charging process.

Fig. 7 (a) SEM micrographs of Li–Cu coatings electrodeposited on Cu foils obtained from a solution of 0.066 mol L⁻¹ Cu(II) and 0.77 mol L⁻¹ Li(I) in [EMIm][TFSI] with NMP (volume ratio 2 : 1)) at −3.5 V (vs. Pt) and (b) the deposit being immersed into water for 5 min.

4. Conclusions

This study shows that the electrodeposition of Li–Cu alloy coatings with around 50 at.% lithium is feasible in [EMIm][TFSI] with NMP. The Cu(II) species that are necessary to prepare the coatings must be introduced into the ionic liquid by additive NMP. The content of Li in the Li–Cu deposits can be controlled by the applied potential and the temperature. Based on the analysis of XRD and XPS, the lithium and copper in Li–Cu coating exist as independent metals. It is clearly seen that the copper frame is retained after dissolving the deposited lithium in distilled water.

Acknowledgements

This work is supported by Project (No. 51074057) of National Nature Science Foundation of China.

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