Ionic Liquid Interface and the

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Anion Effects on the Solid/ionic Liquid Interface and the Electrodeposition of Zinc Zhen Liu, Tong Cui, Tianqi Lu, Maryam Shapouri Ghazvini, and Frank Endres J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07812 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 27, 2016

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The Journal of Physical Chemistry

Anion Effects on the Solid/Ionic Liquid Interface and the Electrodeposition of Zinc Zhen Liu,* Tong Cui, TianQi Lu, Maryam Shapouri Ghazvini, Frank Endres* Institute of Electrochemistry, Clausthal University of Technology, Arnold-Sommerfeld-Strasse 6, 38678 Clausthal-Zellerfeld, Germany. E-mails: [email protected], Tel.: +49 5323 72 2494 [email protected], Tel.: +49 5323 72 3141. Abstract The effect of anions on the solid/ionic liquid (IL) interface and the electrodeposition of

zinc

have

been

investigated.

The

employed

ILs

are

composed

of

1-ethyl-3-methylimidazolium ([EMIm]+), bis(trifluoromethylsulfonyl)imide (TFSI−), trifluoromethylsulfonate (TfO−), methylsulfonate (OMs−) and acetate (OAc−), respectively. These anions show an increasing cation–anion interaction strength in the order TFSI− < TfO− < OMs− < OAc−, as probed by far infrared spectroscopy below 200 cm−1. It was shown by in situ AFM that the anion has a profound impact on the interfacial properties. Multilayered structures were observed at the electrode/IL interface for [EMIm]TFSI and [EMIm]TfO, respectively, while only a few layers with rather a low push-through force were found at the interface for [EMIm]OMs and [EMIm]OAc, respectively. The coordination of Zn(II) ions in these ILs by varying zinc salts was investigated by Raman spectroscopy. The differences in metal species and interfacial layers have a strong influence on the electrochemical process and on the quality of the deposits. Dense zinc deposits with nanowire-like and hexagonal plate-like structures were obtained from ILs with TFSI− and TfO− anions, respectively. 1

ACS Paragon Plus Environment


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Thin layers of zinc with porous and spongy structures were obtained in ILs with OMs− anion containing 0.2 mol/L zinc salts, while homogenous and smooth deposits with a fine-grained structure were obtained with 1 mol/L zinc salts. However, no deposits were found in Zn(OAc)2/[EMIm]OAc under the same conditions. These results indicated that the anions of ILs strongly affected the solid/IL interface, the speciation of Zn(II) ions in ILs and the morphology of zinc deposits. Introduction Metallic zinc is a promising anode material for rechargeable batteries due to its good specific capacity, abundance, low-cost, non-toxicity and ease of handling.1-2 However, the challenge of dendritic growth of zinc during charging, especially in KOH solutions, has to be addressed, as dendrites lead to capacity fading or even short circuiting of the batteries. Ionic liquids (ILs) and their mixtures with molecular solvents are of great interest, as they have good electrical conductivities and wide electrochemical windows.3-5 Although the electrodeposition of zinc has been previously carried out in ILs with discrete anions,6-9 the deep understanding of the relationship between cation–anion combinations and their performance as solvents in the electrochemical deposition process is still low. The properties and structures of ILs depend on size, shape and chemical compositions of both cations and anions.10 The properties such as viscosity, conductivity and melting point are determined by the intermolecular interactions between cations and anions and can be tuned by suitable cation–anion combinations.11-13 Cation–anion interactions also dominate the structures of ILs both 2


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in the bulk phase and at the solid/IL interface.14-15 The structure at the solid/IL interface is different from that of the bulk phase. At the solid/IL interface, charge transfer and mass transport processes take place, which drive electrochemical reactions.16 The structures, properties and dynamics of the solid/IL interface play a significant role in electrochemical deposition processes.17-18 The cation of the IL has been shown to have a great effect on the interfacial layer properties19 and on the morphology of the deposits.6,

20

The electrodeposition of zinc in ILs with

pyrrolidinium cations lead to nanocrystalline deposits, while in ILs with imidazolium cations rather microcrystalline deposits are obtained.6 In contrast to cation effects on the interfacial properties and the electrodeposition processes, there is only little knowledge on the influence of the anion. The interaction between metal ions and anions of ILs governs the speciation and also influences the solubility. The speciation of metal ions in ILs is also an important issue, as the speciation will affect the redox potentials and the electrochemical kinetics.21 Raman spectroscopy seems to be an easy and effective tool to study the speciation of metal ions in ILs. The vibrational modes of some weakly coordinating anions, such as TFSI−, TfO− and FSI− anions are very sensitive to the coordination environment, which shift their peak positions and change the intensities upon coordination. The complex environments of magnesium,22-23 lithium,24 sodium,25 and zinc ions26 in ILs, studied by Raman spectroscopy, were reported in literature. In our previous paper, we examined the effect of the IL cation on the zinc speciation and on the morphologies of the zinc deposits.27 In this study, we investigate 3



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the effect of anions on the structure of the IL, the solid/IL interface, the speciation of Zn(II) ions in ILs, their electrochemical performances, and the morphologies of zinc deposits. We suggest routes to design suitable electrolytes for battery applications. Experimental The ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]TFSI,

99%),

1-ethyl-3-methylimidazolium

trifluoromethylsulfonate

([EMIm]TfO, 99%), 1-ethyl-3-methylimidazolium methylsulfonate ([EMIm]OMs, 98%) and 1-ethyl-3-methylimidazolium acetate ([EMIm]OAc, 98%) were purchased from IO-LI-TEC, Germany. The zinc salts, zinc trifluoromethylsulfonate (Zn(TfO)2, 99%) and zinc methylsulfonate, (Zn(OMs)2, 98%), were obtained from IO-LI-TEC as well. Zinc bis(trifluoromethylsulfonyl)imide (Zn(TFSI)2, 99%) and zinc acetate (Zn(OAc)2, 99%) were purchased from Sigma Aldrich, Germany. The cyclic voltammetry and electrodeposition experiments were carried out in an argon filled glove box (OMNI-LAB from Vacuum Atmospheres) on a PARSTAT 2263 potentiostat/galvanostat controlled by PowerCV and PowerStep software. The electrochemical cell made of polytetrafluoroethylene (Teflon) was clamped over a Teflon covered Viton o-ring, thus yielding a geometric surface area of 0.3 cm2. Gold substrates (gold on glass) from Arrandee Inc., predominantly Au(111), were used as working electrodes for our fundamental studies. A platinum wire (Alfa, 99.99%) was used as counter electrode and a zinc wire was used as quasi-reference electrode, respectively. The morphologies of the deposits were characterized by SEM (JSM 7610F, JEOL). 4


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X-ray diffraction patterns were recorded using a PANalytical Empyrean Diffractometer with Cu Kα radiation. The Far-IR measurements were performed on a Bruker VERTEX 70 FT-IR spectrometer equipped with an extension for measurements in the far infrared region, which consists of a multilayer mylar beam splitter and a room temperature DLATGS detector with preamplifier. The Raman measurements were carried out with a Raman module FRA 106 (Nd:YAG laser, 1064 nm) attached to a Bruker IFS 66 v interferometer with a resolution of 2 cm−1. Atomic force microscopy (AFM) experiments were performed using a Molecular Imaging PicoPlus AFM in contact mode. A silicon SPM-sensor from Nano World was used for all experiments. Results and discussion Structure, interaction and hydrogen bonds in ILs The structure and properties of ILs are largely determined by the interaction between cation and anion of the respective IL. Vibrational spectroscopy is a useful tool to investigate the structure and hydrogen bonds in ILs.28-29 Far infrared spectra of [EMIm]TFSI, [EMIm]TfO, [EMIm]OMs and [EMIm]OAc in the range between 30 and 300 cm−1 are shown in Figure 1. The low-frequency vibrational bands below 200 cm−1 can be assigned to the bending and stretching modes of the cation−anion interaction represented by the +C-H···A (A: anion) hydrogen bonds in ILs.30 The peaks indicated with arrows in Figure 1 shifted to higher wavenumbers by varying the anion, corresponding to the strength of the interaction between cation and anion 5



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increasing in the order TFSI− < TfO− < OMs− < OAc−. The OAc− anion shows the strongest hydrogen bonds with [EMIm]+ cation. This is not surprising as the IL with acetate anion can even dissolve cellulose.31

Figure 1. Left: Far-IR spectra of [EMIm]TFSI, [EMIm]TfO, [EMIm]OMs and [EMIm]OAc between 30 and 300 cm−1. Right: Chemical structures of cation and anions. Consequently, the structures of the [EMIm]+ cation must be slightly different in these ILs as the C(2)H and C(4/5)H of the cation are involved in hydrogen bonds with the anions, leading to different bond lengths. It was reported that the structural differences of ILs can be revealed by FTIR spectroscopy.28-29 Figure 2 shows the FTIR spectra of these ILs in the C-H stretching region at wavenumbers between 2500 cm−1 and 3500 cm−1. The peaks between 2500 and 3050 cm−1 are attributed to the CH modes of the methyl and ethyl groups of the [EMIm]+ cation.32-33 These peaks are almost unchanged by varying the anions as they are not involved in hydrogen bonds. The peaks between 3050 and 3200 cm−1 are assigned to the stretching modes of 6


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C(2)H, C(4)H and C(5)H of the [EMIm]+ cation.32-33 These peaks gradually shift to lower wavenumbers by varying the anions from TFSI− to OAc−, as show in Figure 2, indicating a strengthening in the hydrogen bonds between C(2, 4, 5)H and the anions and a lengthening of the C(2)-H and C(4/5)-H bonds. The FTIR results clearly indicated that the structures of the ILs in the bulk liquid were altered by varying the anion.

Figure 2. FTIR spectra of [EMIm]TFSI, [EMIm]TfO, [EMIm]OMs and [EMIm]OAc between 2500 and 3500 cm−1. Cyclic voltammetry of neat ILs The cyclic voltammetry (CV) of [EMIm]TFSI, [EMIm]TfO, [EMIm]OMs and [EMIm]OAc on gold substrates are shown in Figure 3. The electrode potential was ramped down from the open circuit potential (OCP) to −1.5 V or −1.0 V (for [EMIm]OAc) in the forward scan and then up to +2.5 V in the reverse scan with a scan rate of 10 mV/s. In the cathodic regime, [EMIm]TFSI shows one reduction peak 7



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c1, prior to the decomposition of the IL at ~ −1.5 V (c2), which can be related to the adsorption of [EMIm]TFSI ions on the gold surface and/or the formation of solvation layers. A similar behavior was found for [EMIm]TfO, except that two reduction peaks (c1 and c2) are observed, prior to the breakdown of the IL at c3. In the case of [EMIm]OMs, the cathodic process c2 (start at ~ −1.0 V) and the anodic process a2 are related to the decomposition of the [EMIm]OMs ions and the oxidation of the decomposed products, respectively, evidenced by comparing the two curves (black and red) reversed at different potentials. The cathodic peak c1, started at 0.0 V and centered at −0.5 V, might attribute to the adsorption of [EMIm]OMs ions on the gold surface. In [EMIm]OAc, only one reduction peak is found and the onset of the reduction occurs at −0.6 V (c1). It is clearly shown that the electrochemical stabilities of these ILs in the cathodic regime decrease by varying the anion from TFSI− to OAc−. Although they have the same [EMIm]+ cation, the structural differences of the ILs as indicated by FTIR spectroscopy, affects the electrochemical windows. We can't exclude the possibility that the presence of impurities, e.g. water in ILs, decrease the electrochemical window as the quality of [EMIm]OMs and [EMIm]OAc (98%) is slightly lower than that of [EMIm]TFSI and [EMIm]TfO (99%). In addition, the anion can also influence the electrochemical stability by adsorption on the surface and being reduced during the cathodic scan. It was reported that the reduction of TFSI− anion occurs during cathodic polarization.34-35 The TfO−, OMs− and OAc− anions are less electrostable than TFSI− anion. Therefore, they are also expected to decompose under cathodic polarization. 8


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Figure 3. Cyclic voltammograms of neat [EMIm]TFSI, [EMIm]TfO, [EMIm]OMs, and [EMIm]OAc on a gold substrate at room temperature. Scan rate: 10 mV/s. Interfacial structures at the electrode/IL interface To assess the influence of the IL anion on the electrode/IL interface, in situ AFM measurements were performed on these ILs as a function of electrode potentials on Au(111). In the investigated potential regime from the open circuit potential (OCP) to −1.0 V, similar, but not identical, force–distance profiles were observed for the investigated ILs. Representative force–distance profiles at −0.5 V are presented in Figure 4. The data gives information on the surface interaction of the ILs with the electrode surface. The width of each step is indicative of the ionic composition of the interfacial layer confined between the cantilever and the electrode surface. The height of each step represents the force that is required for the cantilever to rupture this layer.36-38 The arrangement of IL ions at the solid/IL interface changes significantly upon varying the anion. In Figure 4a, the pure IL [EMIm]TFSI shows four discrete 9



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steps. The width of the innermost layer is ~ 0.67 nm, followed by three layers corresponding to the presence of [EMIm]TFSI ion pairs, which has a diameter of 0.75 nm estimated by assuming a cubic packing geometry. The reduced width of the innermost layer is possibly a result of the cation being orientated more parallel to the surface, as suggested by Atkin et al.19 For [EMIm]TfO, the IL also adopts a multilayered structure as shown in Figure 4b. The step width of 0.7–0.8 nm is consistent with the size of [EMIm]TfO ion pair, which has a diameter of 0.68 nm. Upon changing the anion to OMs− in Figure 4c and to OAc− in Figure 4d, fewer steps are observed and the forces required to rupture the layers decrease a little. Spectroscopic results revealed that the interaction strength between [EMIm]+ cation and the anions is in order of TFSI− < TfO− < OMs− < OAc−. This indicated that hydrogen-bond networks are more favorable in ILs with OMs− and OAc− anions than that with TFSI− and TfO− anions. Therefore, [EMIm]OMs and [EMIm]OAc pack less effectively into solvation layers than [EMIm]TFSI and [EMIm]TfO do. In addition, the separations of the innermost layer were reduced to ~ 0.60 nm for OMs− and to ~ 0.53 nm for OAc−, respectively. Beyond this layer, a second solvation layer was probed in both cases with steps at ~ 1.4 nm (Figure 4c) and at ~ 1.15 nm (Figure 4d), consistent with the diameters of [EMIm]OMs and [EMIm]OAc ion pairs, respectively. The AFM results clearly indicate that the IL anion has a strong influence on the interfacial structures at the electrode/IL interface.

10


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Figure 4. Force versus apparent separation profiles for a silica cantilever approaching a Au(111) surface in [EMIm]TFSI, [EMIm]TfO, [EMIm]OMs and [EMIm]OAc, respectively, at −0.5 V. Speciation of Zn(II) ions in ILs The zinc salts employed in this study are Zn(TFSI)2, Zn(TfO)2 and Zn(OMs)2, and the ILs are [EMIm]TFSI, [EMIm]TfO and [EMIm]OMs. Here, we focus on the speciation of zinc salts in ILs having the same anion, and of the same salts in different ILs to demonstrate the complexation of Zn(II) ions. Raman spectroscopy was employed to assess the complex environments of zinc salts in these ILs. Figure 5 shows the Raman spectra in the region between 720 and 800 cm−1, which are attributed to the 11



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vibrational mode of the anions. These signals are very sensitive to the coordination environments. In the case of zinc salts in ILs having the same anion, i.e. Zn(TFSI)2/[EMIm]TFSI, Zn(TfO)2/[EMIm]TfO, and Zn(OMs)2/[EMIm]OMs, the investigation of the speciation by Raman spectra is quite well established. The peaks at 741, 757 and 770 cm−1 are assigned to free TFSI−, TfO− and OMs− anions, respectively. Upon addition of zinc salts to the corresponding ILs, new peaks appear at higher wavenumbers (as indicated by arrows in Figure 5), coupled with a loss of intensity of the free anions, a consequence of binding anions to Zn(II) ions. The speciation of the zinc salts in ILs having different anions is more complicated than that having the same anion. Upon addition of 0.2 mol/L Zn(TFSI)2 to [EMIm]TfO, three peaks was observed in the Raman spectrum (dark blue curve, Figure 5). The peaks at 741 and 757 cm−1 are attributed to free TFSI− and TfO− anions. The peak at 766 cm−1, seen as a shoulder, is assigned to the coordination of Zn(II) ions with TfO− anions. Surprisingly, the TFSI− anions do not seem to coordinate with Zn(II) ions in Zn(TFSI)2/[EMIm]TfO as there is a strong signal for free TFSI. A similar behavior was found for 0.2 mol/L Zn(TFSI)2/[EMIm]OMs (black curve, Figure 5) and for 0.2 mol/L Zn(TfO)2/[EMIm]OMs (not shown here). Based on these results, the following solvation mechanism for the complexation of Zn(TFSI)2 in [EMIm]TfO is proposed. Zn(TFSI)2(s) + mTfO−(IL) → [Zn(TfO)m](m-2)−(IL) + 2TFSI−(IL) It seems that the speciation of Zn(II) ions in ILs largely depends on the solvation power of the anion and Zn(II) ions are more likely in association with stronger coordinating anions in the mixtures. In the investigated zinc salts and ILs, the strength 12


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of the interaction between Zn(II) ions and the anion is in the order TFSI− < TfO− < OMs−.

Figure 5. Raman spectra of [EMIm]TFSI, [EMIm]TfO, [EMIm]OMs and these ILs containing 0.2 mol/L zinc salts, respectively, in the regime of 720–800 cm−1. Electrochemical performances and morphologies of the deposits The influence of the IL anion on the electrochemical performance and on the morphology of the deposits has also been investigated. The CVs of neat [EMIm]TFSI along with 0.2 mol/L Zn(TFSI)2 are presented in Figure 6a. In the investigated potential range, the IL [EMIm]TFSI is quite stable. The decomposition of the [EMIm]+ cation starts at −1.5 V vs. Zn quasi-reference electrode on a gold substrate. In 0.2 mol/L Zn(TFSI)2/[EMIm]TFSI, the onset of Zn deposition occurs at −0.3 V and shows a reduction peak at −0.65 V. In the backward scan, a current loop was observed 13



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indicating a nucleation process. The oxidation current gradually increases and an oxidation peak forms at 0.5 V, followed by a sharp decrease in the current. This process is attributed to the stripping of zinc. Electrodeposition at −0.5 V for 2 h results in zinc films, which were subsequently characterized by SEM (Figures 6b and c). Nanowire-like structures, which twisted and turned in random directions, were obtained. Template-free deposition of metal and semiconductor nanowires in ILs has been demonstrated.39-42 The deposition parameters such as concentration, temperature and potential have a great influence on the morphology, and both the mass transport and surface passivation seem to play important roles in the formation of nanowires in ILs.39-42

14


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Figure 6. a) CVs of pure [EMIm]TFSI and [EMIm]TFSI containing 0.2 mol/L Zn(TFSI)2. Scan rate: 10 mV s−1. b) and c) SEMs of zinc deposits obtained from 0.2 mol/L Zn(TFSI)2/[EMIm]TFSI at −0.5 V for 2 h at low and high magnifications.

15



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The CVs of neat [EMIm]TfO, 0.2 mol/L Zn(TfO)2/[EMIm]TfO and 0.2 mol/L Zn(TFSI)2/[EMIm]TfO are compared in Figure 7a. In the cathodic scan, the onset of zinc deposition in both cases occurs at −0.15 V. Three reduction peaks, c1, c2 and c3, are observed in 0.2 mol/L Zn(TFSI)2/[EMIm]TfO. All these peaks are attributed to the deposition of zinc. The potential differences might result from the dynamics of the IL interfacial layers as two different anions are present in the electrolytes. In the case of 0.2 mol/L Zn(TfO)2/[EMIm]TfO, similar reduction peaks, c1 and c2, are seen in the forward scan, as Raman spectra evidenced that the species present in the electrolytes are probably the same. In addition, an obvious reduction peak is also seen in the backward scan (black curve, Figure 7a), which is attributed to further zinc deposition in the presence of an IL passivation layer. The passivation layer mainly comprised of ionic liquid cations and anions and/or their decomposed products. At negative potentials, the number of solvation layers increases and the interaction strength become stronger, which might limit the mass transport of Zn(II) species. In the positive scan, the ionic liquid passivation layer formed on the surface at negative potentials was released in the positive scan, resulting in further zinc deposition. In the anodic scan, the oxidation peaks in both cases are due to the stripping of zinc. The morphologies of the deposits obtained from these two electrolytes held at −0.5 V for 2 h are shown in Figures 7b and c, respectively. Hexagonal plate-like zinc structures with the plates perpendicular to the substrate are obtained in 0.2 mol/L Zn(TfO)2/[EMIm]TfO (Figure 7b). Cauliflower-like structures made of closely packed thin plate are seen in Figure 7c. 16


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Figure 7. a) CVs of pure [EMIm]TfO and [EMIm]TfO containing 0.2 mol/L Zn(TfO)2 and 0.2 mol/L Zn(TFSI)2, respectively. Scan rate: 10 mV s−1. b) and c) SEMs of zinc deposits obtained from 0.2 mol/L Zn(TfO)2/[EMIm]TfO and from 0.2 mol/L Zn(TFSI)2/[EMIm]TfO at −0.5 V for 2 h, respectively. Figure 8a displays the CVs of pure [EMIm]OMs and [EMIm]OMs containing Zn(TfO)2, Zn(TFSI)2 and Zn(OMs)2, respectively, with a concentration of 0.2 mol/L. Unfortunately, the peaks between zinc reduction and the adsorption of the IL on the surface can not be distinguished as a result of the low zinc deposition current. However, by electrolysis at −0.6 V for 2 h, thin zinc films were obtained in all cases. This indicated that the zinc species are electroactive and can be reduced to metallic zinc. In order to shed more light on the electrochemical behavior, the concentrations of zinc salts were increased to 1 M, and the respective CVs are shown in Figure 8b. The CVs show significant differences in terms of zinc deposition potentials as well as 17



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the deposition and stripping current densities by varying zinc salts. For the CV of 1 M Zn(OMs)2/[EMIm]OMs, only a small reduction peak is noted at −0.8 V during the reverse scan (C*), attributed to the reduction of zinc in the presence of an IL passivation

layer.

The

same

phenomenon

was

found

for

1

mol/L

Zn(TfO)2/[EMIm]OMs, but with higher deposition and stripping current densities. Such a phenomenon has also been

reported in the deposition of zinc from

dicyanamide (dca−) anions containing different zinc salts including Zn(dca)2, ZnCl2, ZnSO4 and Zn(OAc)2.7 The CV of 1 mol/L Zn(TFSI)2/[EMIm]OMs shows two zinc reduction peaks, c1 and c2, in the forward scan. The peak c1 at −0.25 V might due to the under potential deposition (UPD) of zinc and c2 is the bulk deposition of zinc. In addition, the onset of zinc deposition occurs at −0.75 V, −0.6 V and −0.45 V for zinc salts with OMs−, TfO− and TFSI− anions, respectively. The Raman spectra in Figure 4 have revealed that the zinc ions mainly coordinate with OMs− anions forming complexes in all the three electrolytes. In the cases of Zn(TFSI)2 and Zn(TfO)2 in [EMIm]OMs, the free TFSI− or TfO− anions in solution might alter the IL [EMIm]OMs interfacial properties, which favors zinc deposition, or the physical properties of the [EMIm]OMs IL such as viscosity and conductivity was improved in the presence of TFSI− or TfO− anions.

18


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Figure 8. CVs of pure [EMIm]OMs and [EMIm]OMs containing Zn(TfO)2, Zn(TFSI)2 and Zn(OMs)2,respectively, with concentrations (a) at 0.2 mol/L and (b) at 1 mol/L. The morphologies of the deposits obtained from [EMIm]OMs containing different zinc salts are shown in Figure 9. In 0.2 mol/L Zn(TFSI)2/[EMIm]OMs, porous zinc films with a good adhesion to the substrate were obtained (Figure 9a). Spongy-like zinc morphology with a poor adhesion to the gold electrode was seen in 0.2 mol/L Zn(TfO)2/[EMIm]OMs (Figure 9b). Cauliflower-like compact zinc films were obtained in 0.2 mol/L Zn(OMs)2/[EMIm]OMs (Figure 9c). By varying zinc salts with anions from TFSI−, TfO− to OMs−, the morphologies become more compact. At high concentrations (1 mol/L), the morphologies of the deposits were quite different from that obtained at low concentrations (0.2 mol/L). A bright zinc film with a fine nanocrystalline structure as well as some large particles randomly distributed on the surface was obtained in 1 mol/L Zn(TFSI)2/[EMIm]OMs (Figure 9d). The surface of the deposits obtained in 1 mol/L Zn(TfO)2/[EMIm]OMs is quite smooth and homogenous comprised of fine-grained zinc nanoparticles (Figure 9e). A dark gray 19



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zinc coating was obtained in 1 mol/L Zn(OMs)2/[EMIm]OMs (Figure 9f). At high magnification as shown in the inset of Figure 9f, porous zinc structures together with some islands were observed. Compared to ILs with TFSI− and TfO− anions, the surface of the zinc deposits obtained from the IL with OMs− anions is smoother and more homogenous. It seems that the formation of an IL passivation layer during zinc deposition can effectively improve the quality of zinc deposits. The IL interfacial layer strongly depends on the electrode potential and it can influence the electrochemical reactions.43 The formation of a suitable liquid electrolyte interphase at the electrode surface by adjusting the electrode potentials might change the mass transport of Zn(II) species, leading to smooth surface. This phenomenon is – in our opinion – worth of further investigations as it might solve the problem associated with dendritic growth during charging.

Figure 9. SEMs of zinc deposits obtained from [EMIm]TfO containing Zn(TFSI)2 (a, d), Zn(TfO)2 (b, e) and Zn(OMs)2 (c, f), respectively, with concentrations (a-c) at 0.2 mol/L and (d-f) at 1 mol/L. 20


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The electrodeposition of zinc from [EMIm]OAc containing various concentrations of Zn(OAc)2 was also investigated.44 Up to 4 mol/L of Zn(OAc)2 in [EMIm]OAc, no zinc deposition was observed at room temperature. It seems that the species present in the solution is electrochemically unreactive even at relatively high concentrations. Spectroscopy evidences that cation–anion interactions are stronger in [EMIm]OAc than that in [EMIm]OMs. Therefore it is also plausible that a strong IL passivation layer was formed on the surface and inhibited zinc deposition in [EMIm]OAc. Conclusions In conclusion, we have demonstrated that the anion of an IL has a strong influence on the solid/IL interface. Due to the different interactions between cation and anion, the structures both in the bulk phase and at the electrode/IL interface are altered by varying the anions. The speciation of Zn(II) ions in ILs was investigated by Raman spectroscopy. The coordination power of ILs also strongly depends on the strength of the interaction between Zn(II) ions and anions. The electrochemical performance was studied by cyclic voltammetry and the morphologies of the deposits were investigated by SEM. Nanowire-like zinc structures were obtained in ILs with TFSI– anions. Hexagonal plate-like structures were observed in ILs with TfO– anions. Porous spongy-like zinc structures were obtained in ILs with OMs– anions. Upon increasing the concentration of zinc salts, homogenous and smooth deposits with fine-grained structures were obtained in ILs with OMs– anions. The formation of an IL passivation layer is critical in obtaining smooth zinc deposits. Such findings might provide valuable insights into designing ILs as electrolytes to deposit dendrite-free metals, e.g. 21



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for energy storage applications.

Acknowledgement Financial support by the BMBF project LUZI (BMBF: 03SF0499A) is gratefully acknowledged. We would like to thank Mrs. Silvia Löffelholz for the SEM investigations and Mrs. Karin Bode from the Institute of Inorganic and Analytical Chemistry, Clausthal University of Technology, for IR and Raman measurements. References (1) Li, Y.; Dai, H. Recent Advances in Zinc-Air Batteries. Chem. Soc. Rev. 2014, 43, 5257-5275. (2) Parker, J. F.; Chervin, C. N.; Nelson, E. S.; Rolison, D. R.; Long, J. W. Wiring Zinc in Three Dimensions Re-Writes Battery Performance-Dendrite-Free Cycling. Energy Environ. Sci. 2014, 7, 1117-1124. (3) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Ionic-Liquid Materials for the Electrochemical Challenges of the Future. Nat. Mater. 2009, 8, 621-629. (4) MacFarlane, D. R.; Tachikawa, N.; Forsyth, M.; Pringle, J. M.; Howlett, P. C.; Elliott, G. D.; Davis, J. H.; Watanabe, M.; Simon, P.; Angell, C. A. Energy Applications of Ionic Liquids. Energy Environ. Sci. 2014, 7, 232-250. (5) MacFarlane, D. R., et al. Ionic Liquids and Their Solid-State Analogues as Materials for Energy Generation and Storage. Nat. Rev. Mater. 2016, 1, 15005. (6) Liu, Z.; Zein El Abedin, S.; Endres, F. Electrodeposition of Zinc Films from Ionic Liquids and Ionic Liquid/Water Mixtures. Electrochim. Acta 2013, 89, 635-643. (7) Simons, T. J.; Torriero, A. A. J.; Howlett, P. C.; MacFarlane, D. R.; Forsyth, M. High Current Density, Efficient Cycling of Zn2+ in 1-Ethyl-3-methylimidazolium Dicyanamide Ionic Liquid: the Effect of Zn2+ Salt and Water Concentration. Electrochem. Commun. 2012, 18, 119-122. (8) Kar, M.; Winther-Jensen, B.; Forsyth, M.; MacFarlane, D. R. Chelating Ionic Liquids for Reversible Zinc Electrochemistry. Phys. Chem. Chem. Phys. 2013, 15, 7191-7197. (9) Xu, M.; Ivey, D. G.; Xie, Z.; Qu, W. Electrochemical Behavior of Zn/Zn(II) Couples in Aprotic Ionic Liquids Based on Pyrrolidinium and Imidazolium Cations and Bis(trifluoromethanesulfonyl)imide and Dicyanamide Anions. Electrochim. Acta 2013, 89, 756-762. 22


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TOC

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Ionic Liquid Interface and the

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