Suppressing the dendritic growth of zinc in an ionic liquid containing cationic and anionic zinc complexes for battery applications†

Metallic zinc is a promising negative electrode for high energy rechargeable batteries due to its abundance, low-cost and non-toxic nature. However, the formation of dendritic zinc and low Columbic efficiency in aqueous alkaline solutions during charge/discharge processes remain a great challenge. Here we demonstrate that the dendritic growth of zinc can be effectively suppressed in an ionic liquid electrolyte containing highly concentrated cationic and anionic zinc complexes obtained by dissolving zinc oxide and zinc trifluoromethylsulfonate in a protic ionic liquid, 1-ethylimidazolium trifluoromethylsulfonate. The presence of both cationic and anionic zinc complexes alters the interfacial structure at the electrode/electrolyte interface and influences the nucleation and growth of zinc, leading to compact, homogeneous and dendrite-free zinc coatings. This study also provides insights into the development of highly concentrated metal salts in ionic liquids as electrolytes to deposit dendrite-free zinc as an anode material for energy storage applications.


Introduction
Rechargeable zinc-based batteries, including Zn-air, Zn-Ni and Zn-MnO 2 batteries, are promising energy storage devices with high energy density, safety and economic feasibility. [1][2][3] One of the main challenges that these rechargeable battery systems faces is the intrinsic properties of zinc to form dendrites during charging. These dendrites grow in subsequent deposition/stripping cycles, resulting in short circuits of the cell. Although intensive efforts have been made on preventing the formation of dendrites in aqueous and organic electrolytes, including modification of the separator, 4 additives in the electrode/electrolyte, [5][6][7][8][9][10][11] and adjustment of the deposition parameters, 12 the low Columbic efficiency, hydrogen evolution in aqueous solution and safety concerns in organic solvents have limited their large-scale applications. The performance of the electrolyte is critical in the cycling stability of Zn anodes. The structure of the electrode/electrolyte interface has a strong influence on electrochemical reactions, 13,14 as both charge transfer and mass transport processes take place at the electrode/electrolyte interface. Therefore, the exploration of an alternative non-aqueous electrolyte that can lead to long-life, high rate and stable cycling of the zinc anode is critical for the development of rechargeable zinc metal batteries.
Ionic liquids (ILs) are low-temperature molten salts composed entirely of ions. [15][16][17] ILs generally contain large asymmetric cations and weakly coordinating anions. 18 They have some advantages over aqueous-based and organic solvents as electrolytes for the electrodeposition of metals and semiconductors due to their large electrochemical windows, extremely low vapor pressures and good conductivity. [19][20][21][22] One of the disadvantages is that metal salts tend to be poorly soluble in ILs with weakly coordinating anions, such as trifluoromethylsulfonate (TfO − ), bis(trifluoromethylsulfonyl)imide (TFSI − ), tetrafluoroborate (BF 4 − ) and hexafluorophosphate (PF 6 − ). 23 Alternatively, the solubility can be improved by replacing weakly coordinating anions with stronger coordinating ones like chloride, but the melting points and viscosities can increase. 24 Examples for such types of liquids are composed of anions viz. Al 2 Cl 7 − or ZnCl 3 − /ZnCl 4 2− with a combination of AlCl 3 or ZnCl 2 and 1,3-dialkylimidazolium chlorides. By the combination of a cation-anion of an appropriate design, the physical and chemical properties of the ILs can be modified to improve the solubility. It was reported that the solubility could be increased by functionalizing cations of the ILs (e.g. ether-or hydroxyl-functionalized ILs). 25 Other approaches to overcome this drawback are to prepare cationic complexes of metal ion species by coordinating the metal salts with neutral ligands such as tertiary amines, urea and alkyl sulfides. [26][27][28][29] This kind of liquid possesses cationic and anionic metal complexes, for instance, AlCl 3 coordinates with amide, leading to the formation of an ionic compound of the formula [AlCl 2 ·namide][AlCl 4 ]. 27 Another way to synthesize the cationic metal complexes is to incorporate the metal ion as a part of cation of the IL. 30,31 Since the metal ion is a part of the cation, it moves toward the cathode by electromigration under deposition conditions, which gives an advantage over anionic metal species. Another advantage is that such liquids do not possess cathodic decomposition potentials as the reduction reaction is the deposition of metal. The incorporation of metal ions in the cations of ILs gives a high concentration of metal ions, which allows an increased rate of metal deposition.
High-rate electrodeposition of zinc from ILs with N-alkylimidazole ligands coordinated with Zn 2+ cations and TFSI − anions was described by Steichen et al. 31  In the present paper, an IL electrolyte containing both cationic and anionic zinc complexes has been explored to address these challenges. The IL electrolyte was synthesized by dissolving zinc oxide and zinc trifluoromethylsulfonate (Zn(TfO) 2 ) in a protic IL, 1-ethylimidazolium trifluoromethylsulfonate ([EIm]TfO). The highly concentrated electrolyte composed of both cationic and anionic zinc complexes results in dendritefree Zn deposits and a good cycling efficiency at room temperature. The interfacial properties of this IL and the nucleation/ growth of zinc are also addressed.

Speciation of Zn 2+ in ionic liquids
Raman spectroscopy has been employed to investigate the complex environment of zinc salts in the ionic liquid. The  Fig. 1, which indicates that the imidazolium cation converted to imidazole upon reaction with ZnO. Furthermore, the geometry of the 1-ethylimdazolium ring may also alter upon reaction with ZnO. The ethyl group of the imidazolium ring can rotate along the C-N bond, yielding planar and nonplanar conformers, which give different Raman bands. 39,40 The coordination of Zn 2+ ions with the 1-ethylimidazole ring may shift the equilibrium of these conformers, which are evidenced by a decrease in peak intensity at 1320 cm −1 and an increase in peak intensity at 1240 cm −1 as indicated by arrows in Fig. 1. In addition, a peak shoulder was observed in the regime of 240-210 cm −1 for the spectra of (c) and (d), respectively, which can be due to the stretching vibration of the Zn⋯N bonds. The interaction of Zn 2+ with the imidazolium ring was further evidenced by nuclear magnetic resonance (NMR) spectroscopy.
NMR is a useful tool to investigate the interaction between the cation and the anion of the IL. The chemical shifts (δ) for the proton resonance of the imidazolium ring without and with zinc salts are plotted in Fig. 2. The chemical shifts are found at 9.06, 7.77, 7.65, 4.20, and 1.42 ppm for pure [EIm]TfO, which can be attributed to the proton resonance of C(2)-H, C(4)-H, C(5)-H, -CH 2 and -CH 3 , respectively. 41 In addition, a broad peak with low intensity between 15 and 11 ppm is also observed, which is related to the proton resonance of N-H. The addition of Zn(TfO) 2 to [EIm]TfO leads to a downfield shift of the N-H proton, indicating stronger hydrogen bonding between TfO − and imidazolium cations via N-H. However, the chemical shifts of C(2)-H, C(4)-H and C(5)-H do TfO. In addition, the chemical shift for N-H disappeared, which confirms the formation of Zn⋯N bonds and water upon the reaction of ZnO with the [EIm] + cation. The formed water molecules are not "free" in the solution, rather they interact with both [EIm] + and TfO − by hydrogen bonds via the C(2)-H, C(4)-H and C(5)-H and the -SO 3 group. 34 Thus, NMR spectroscopy gives valuable insights into hydrogen bonding and Zn⋯N bond formation.
Raman spectroscopy is also used to investigate the environment of Zn 2+ with TfO − anions by monitoring the CF 3 symmetric deformation vibrational mode of the anion, δ s (CF 3 ). In pure [EIm]TfO (Fig. 3a), the peak at 759.3 cm −1 originates from the vibrational mode of "free" TfO − anions. 42,43 The peak can be well fitted by a Voigt function. On addition of 0.4 M Zn(TfO) 2 to the IL (Fig. 3b), a new distinct peak centered at 765.3 cm −1 is observed, which is attributed to the vibrational mode of Zn 2+ coordinated TfO − . By integrating the areas of the two curves, the average number of TfO − anions coordinated to Zn 2+ is calculated to be 3.1. 42 This means that each Zn 2+ ion is coordinated by three TfO − anions forming anionic zinc complexes of [Zn(TfO) 3 ] − . In 1.5 M ZnO/IL (Fig. 3c), the frequency is shifted to a higher wavenumber of 761.4 cm −1 and the peak can be well fitted by a Voigt function. This shift is caused by the association of water with TfO − anions. We previously reported that the vibrational mode of δ s (CF 3 ) shifted to higher wavenumbers upon adding water to 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm]TfO). 42 The zinc complexes present in 1.5 M ZnO/[EIm]TfO are cationic [Zn(EIm) 2 ] 2+ , and the Zn 2+ ions do not coordinate with TfO − anions as there is only a slight shift in the free TfO − peak and no new peak/shoulder is observed at higher wavenumbers. In the mixtures of (0.4 M Zn(TfO) 2 + 1.5 M ZnO)/IL, the Raman spectrum in Fig. 3d shows a broad peak at 764.1 cm −1 that can be deconvoluted into three peaks, which are centered at 758.9, 762.5 and 766.0 cm −1 . They are related to free TfO − , coordinated TfO − with water, and coordinated TfO − with Zn 2+ . The average number of TfO − coordinated to Zn 2+ is calculated to be 4. Interfacial structure at the electrode/electrolyte interface The interfacial properties of the electrode/electrolyte interface have been investigated by in situ atomic force microscopy (AFM) as a function of electrode potential. The step-wise forcedistance profiles of the pure IL adopt a multilayered structure instead of a classical electric double layer. [44][45][46][47] The width of each step can be due to the presence of cations, anions, or ion pairs confined between the cantilever and the electrode surface. 48 Thus, the ionic composition of interfacial layers can be obtained by analyzing the widths of the steps. The interfacial structure can be divided into three regions: the innermost layer, the transition zone and the bulk liquid. 49 The innermost layer of ionic species is in direct contact with the electrode surface and shows the highest order. 50     the force-distance at −0.3 V is discussed here. In Fig. 4a, the pure IL shows four discrete steps.  Fig. 4b shows that the width of the innermost layer is lowered to 0.3 nm with a force of 5.5 nN. Also the innermost layer appears to have a gradient with multiple steps. The Raman spectrum in Fig. 3b Fig. 4d. The interfacial layers appear quite wide and rather distorted, showing an average separation of ∼0.80 nm. It appears that more than one species is present in the innermost layer. The interfacial layer cannot be probed at potentials more negative than −0.4 V due to the underpotential deposition of zinc or due to alloying of zinc with gold. Thus, from AFM analysis, it can be concluded that the presence of different zinc species changes the interfacial structure significantly which will lead to different electrochemical processes during zinc deposition/stripping.

Electrochemical behavior
The electrodeposition of Zn on gold from these electrolytes has been studied using cyclic voltammetry and chronoamperometry.  (Fig. 5a, black curve), the onset of Zn 2+ reduction occurs at −1.4 V and shows a reduction peak (c1) at −1.8 V with a current density of ∼3 mA cm −2 . In the backward scan, a current cross over is recorded at −1.3 V. This current loop is indicative of a nucleation process. The oxidation peak (a1) is attributed to the stripping of zinc and the stripping efficiency is ∼95% as calculated by integration of the current densities in the CV. The addition of Zn(TfO) 2 to [EIm]TfO widens the electrochemical window of the electrolyte. Upon addition of ZnO to the IL and to Zn(TfO) 2 /IL, the CVs change their shapes. The reaction of ZnO with the imidazolium cation results in the association of Zn 2+ with 1-ethylimidazole by the replacement of H from the imidazolium cation, which enhances the electrochemical stability of the electrolyte. In addition, the cationic [Zn(EIm) 2 ] 2+ complexes can move towards the cathode during electrodeposition not only by diffusion but also by electric field. In 1.5 M ZnO/IL (Fig. 5a, red curve), the reduction of zinc occurs at −1.45 V and has a lower current density compared to that in 0.4 M Zn(TfO) 2 /IL, because the former has a higher viscosity (262 mPa s at 25°C) than the latter (81 mPa s at 25°C). In   (Fig. 5a, blue curve) looks similar to that of 1.5 M ZnO/IL but with a slightly lower current density. In both cases, no cathodic peak is observed in the investigated potential regime. The results are consistent with previous reports on metal deposition from cationic metal containing ILs, where the authors suggested that the deposition process is not limited by mass transport. 30,31,53 Nucleation and growth kinetics in the initial stages of Zn deposition on gold are studied by chronoamperometry. Fig. 5b shows typical current transients for Zn deposition from these electrolytes at −1.8 V. In 0.4 M Zn(TfO) 2 /IL, the transient is characterized by an initial increase in the current at short times and then a decay (t = 1.2 s). Subsequently there is a rapid rise in the curve to a peak (t = 6.5 s) followed by a gradual decay. The transient exhibits a typical shape for metal deposition involving nucleation and growth. The inset of Fig. 5b presents j versus t −1/2 plot for the descending parts of the transient for 0.4 M Zn(TfO) 2 /IL. As seen, the transient deviates from the Cottrell's behavior. Since the electrode potential is in the range of the hydrogen evolution potential (Fig. 4, cyan curve), it is expected that proton reduction simultaneously occurred during Zn electrodeposition, which affects the diffusion process. However, for ZnO containing electrolytes without and with Zn(TfO) 2 , the deposition mechanism is considerably different. In both cases, the transients are characterized by a sharp increase in the current and then a decay (t = 2.5 s). In the succeeding part of the transient, a rise in the current is recorded at t > 2.5 s and almost constant current is recorded at t > 5 s. Despite the differences in zinc species, zinc concentrations and viscosity, the transients of the two ZnO containing electrolytes are similar. The results suggest that both the cationic and anionic zinc complexes in (0.4 M Zn(TfO) 2 + 1.5 M ZnO)/IL are reduced in the investigated overpotential (−1.8 V). A synergetic effect may exist between cationic and anionic zinc species in the nucleation and growth of zinc.

Morphology of the zinc deposits
Earlier results in the literature showed that zinc dendritic growth preferentially occurs at the edges and along the boundaries of the electrode where the current density is higher compared to the center. 54 Fig. 6. The zinc deposits obtained from 1.5 M ZnO/IL show dendrites which are perpendicular to the surface both at the edges and the center region of the deposits (Fig. 6a and b). For the zinc deposits obtained from 0.4 M Zn(TfO) 2 /IL, granular zinc particles were seen at the edges (Fig. 6c) while compact and dense zinc deposits with a hexagonal structure were observed in the center region (Fig. 6d). The [EIm] + cations can be decomposed and they can generate hydrogen bubbles during zinc deposition and edges are the preferential sites for hydrogen embrittlement. Therefore, granular zinc aggregates with poor adhesion to the substrate were found at the edges. In contrast, compact and dendrite-free zinc was successfully electrodeposited from (0.4 M Zn(TfO) 2 + 1.5 M ZnO)/IL (Fig. 6e and f ). The deposits are bright and have a uniform crystal size. A hexagonal-shaped zinc structure can be seen in the insets of Fig. 6e and f. In addition, the XRD results (ESI Fig. S2 †) show that for the electrolytes of 1.5 M ZnO/IL and 0.4 M Zn(TfO) 2 /IL, the zinc deposits have a preferred (101) orientation, while it is a (002) orientation for (0.4 M Zn(TfO) 2 + 1.5 M ZnO)/IL.

Cycling stability
To evaluate the feasibility of the (Zn(TfO) 2 + ZnO)/IL electrolyte towards practical applications, a current density of 2 mA cm −2 is applied for cycling tests. The cycling behavior of zinc in 0.4 M Zn(TfO) 2 /IL and in 1.5 M ZnO/IL has also been investigated for comparison. The voltage profiles are shown in Fig. 7a-c. In 0.4 M Zn(TfO) 2 /IL (Fig. 7a), a charge over-voltage of ∼−0.35 V and a discharge over-voltage of ∼0.4 V are observed, respectively, and they are stable for the initial 50 cycles. With increasing cycle numbers (>100 cycles), a rapid increase in the over-voltage is recorded after 600 s. At these higher over-potentials, decomposition of imidazolium cations and side reactions probably occur, decreasing the Columbic efficiency of the cell and leading to an uneven growth of the deposits. In 1.5 M ZnO/IL (Fig. 7b), the charge voltage profile is not flat, but it gradually increases as charging proceeds. In addition, the charge voltage also gradually increases as the cycling number increases. Dendritic growth of Zn deposits as shown in Fig. 6a and b leads to uneven distribution of the current on electrode's surface, resulting in instability of the voltage. However, the discharge voltage is almost stable at ∼0.95 V for the initial 50 cycles and a rapid rise in the overpotential is observed in the last 200 s after 100 cycles. In (0.4 M Zn(TfO) 2 + 1.5 M ZnO)/IL (Fig. 7c), relatively flat voltage profiles are seen during charge and discharge. The charge voltage is observed in the range of −0.90-−1.1 V in the initial cycles, which gradually polarizes each cycle to −1.1-−1.3 V in the 170 th cycle. The discharge voltage is stable at ∼0.95 V. The cell with (0.4 M Zn(TfO) 2 + 1.5 M ZnO)/IL as the electrolyte is able to cycle well with a stable voltage and supports a reversible reaction. Fig. 7d-i show the optical and SEM images of electrodes after 170 cycles. Several hollow shell like structures are found in Zn(TfO) 2 /IL ( Fig. 7d and g). Obviously, dendritic aggregates are found in ZnO/IL (Fig. 7e and h). The formation of zinc dendrites tends to cause short circuits of the cell. In contrast, for the (Zn(TfO) 2 + ZnO)/IL electrolyte, homogeneous and dendrite-free zinc deposits are obtained ( Fig. 7f and i).

Discussion
The dissolution of metal oxides such as Ag 2 O, NiO, CuO, and ZnO in 1,3-dialkylimidazolium ILs is described in the literature. 56 The dissolution occurs through a deprotonation of C(2)-H of the imidazolium cation, resulting in the formation of Ag-carbene complexes and water. In our case, we have C(2)-H and N-H in the employed protic IL. The question arises here is which 'H' is deprotonated upon the reaction with ZnO. The proton of N-H is more acidic than the C(2)-H of the imidazole ring. Thus, deprotonation preferably occurs at N-H. In the NMR spectra, the proton resonance of N-H after the reaction with ZnO is missing. In the case of Raman spectra, the TfO − anion signal is slightly shifted from 759.3 to 761.4 cm −1 , which can be due to the association of water with TfO − anions. 42 However, we could not find a drastic change in the anion environment for the coordination of TfO − anions with Zn 2+ like in the case of Zn(TfO) 2 /[EIm]TfO in Fig. 3b The zinc morphology in Fig. 6 shows that a dendritic deposit occurs on addition of ZnO whereas a spherical deposit is formed in the presence of Zn(TfO) 2 in the ionic liquid. In the presence of both salts, a very fine Zn deposit is obtained. The changes in the morphology can be associated with the interfacial structure shown in Fig. 4. It is evident that the innermost interfacial layer significantly changes on addition of Zn(TfO) 2 and of Zn(TfO) 2 + ZnO compared to the pure IL, whereas in the case of ZnO addition, no change occurs. This suggests that the change in the innermost layer affects the zinc deposition process. Furthermore, in the case of Zn(TfO) 2 + ZnO, the innermost layer appears to show the presence of more than one species (Fig. 4d) which might have led to the deposition of very fine zinc structures as seen in Fig. 6e and f. The sustainability of the innermost layer might also be a reason for not having dendritic growth as evidenced in Fig. 7g after 170 cycles.
One concern towards practical application of the (Zn(TfO) 2 + ZnO)/[EIm]TfO electrolyte in dendrite-free zinc deposition is its high viscosity of 362 mPa s at 25°C. However, it can be considerably reduced by adding water to the solution, and the solution still acts as an ionic liquid. 42 Several investigations have revealed that upon addition of small amounts of water to ionic liquids, the zinc deposition was even facilitated and smooth zinc deposits were obtained as well as the cycling behavior was improved. [57][58][59]

Conclusion
In conclusion, we have demonstrated that the dendritic growth of zinc can be effectively suppressed in the (Zn(TfO) 2   stable voltage during long-term cycling and dendrite-free zinc is observed at a current density of 2 mA cm −2 in (Zn(TfO) 2 + ZnO)/[EIm]TfO.

Electrochemical measurements
The cyclic voltammetry, chronoamperometry, chronopotentiometry, and galvanostatic charge-discharge test experiments were carried out in an argon-filled glove-box using a VersaStat III (Princeton Applied Research) potentiostat/galvanostat controlled by Power-CV and Power-Step software. The electrochemical cell made of polytetrafluoroethylene (Teflon) was clamped over a Teflon covered Viton O-ring. Prior to the experiments, the Teflon cell and the O-ring were cleaned in a mixture of 50 : 50 vol% of concentrated H 2 SO 4 and H 2 O 2 followed by refluxing in distilled water. Gold substrates (gold on glass) from Arrandee Inc. were used as working electrodes. A platinum wire (Alfa, 99.99%) was used as a counter electrode. Prior to each experiment, the Pt wire was cleaned with isopropanol in an ultrasonic bath for 15 min and annealed in a hydrogen flame to red glow in order to remove any possible contaminations. The reference electrode consists of a silver wire immersed in the employed IL containing 10 mM silver trifluoromethylsulfonate (AgTfO), separated from the bulk solution by a glass frit (G4). Before using, a Nafion solution (LQ-1105 5% by weight NAFION®, 1100 EW) was filled in the glass frit and dried at 100°C for 2 h to inhibit leakage. However, the potentials were referenced to the ferrocene/ferrocenium redox couple (Fc/Fc + 0.54 V vs. Ag/AgTfO) in the experiments. A Zn/electrolyte/Zn symmetric cell was used for the cycling test. A zinc sheet was used as the working electrode and a zinc wire/sheet was used as the reference and counter electrodes, respectively.

Characterization
The deposits were characterized by scanning electron microscopy (Carl Zeiss DSM 982 Gemini). The X-ray diffraction patterns were recorded on a PANalytical Empyrean diffractometer. The Raman measurements were carried out with a Raman module FRA 106 (Nd:YAG laser, 1064 nm) attached to a Bruker IFS 66v interferometer at room temperature. Atomic force microscopy (AFM) experiments were performed using a Molecular Imaging Pico Plus AFM in contact mode. A silicon SPM-sensor from Nano World was used for all experiments. The viscosity was measured on an AR1000/500 rheometer (TA Instrument, USA).