CHAPTER III. Water-Repellent Ionic Liquid Skinny Gels Customized for Aqueous Zinc-Ion Battery Anodes
3.3. Results and Discussion
The fabrication of an IL skinny gel on a Zn anode is schematically depicted in Figure 21. An IL gel precursor (a mixture of Zn(TFSI)2 salt, BMPTFSI solvent, and thiol-ene monomer) was spin-coated directly on top of a Zn metal foil (thickness = 250 mm), followed by UV-irradiation-assisted curing, producing the IL skinny gel. The role of the IL skinny gel as a water-repellent ion-conducting protective layer is conceptually illustrated in Scheme 1. Meanwhile, processing solvents were not used during the fabrication of the IL skinny gel, which eliminated drying steps and resolved concerns about unwanted interfacial side reactions between the processing solvents and Zn metals.
The miscibility of ILs with water is strongly affected by their anion characteristics.145-146 Three ILs with the identical BMP+ cation and different anions were mixed with water at a composition ratio of IL/water = 1/2 (v/v). The BMPTFSI showed the immiscibility with water, whereas the other ILs with tetrafluoroborate (BF4−) and trifluoromethanesulfonate (OTf−) anions were miscible with water (Figure 22a). This difference in the miscibility between the ILs and water was verified by analyzing 19F nuclear magnetic resonance (NMR) spectra (Figure 22b). The integrated peak ratios of BF4− (13.2) and OTf− (10.1) in the IL/water mixtures were well matched with the amounts of the initially added ILs (13.2 for BF4−, 10 for OTf−). In contrast, a characteristic peak corresponding to the TFSI− was hardly detected in
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the water phase of the BMPTFSI/water mixture, exhibiting the crucial role of anions in determining the immiscibility with water.
Figure 21. Schematic illustration depicting the fabrication of an IL skinny gel on a Zn anode, along with its chemical structure and role as a water-repellent ion-conducting protective layer.
Figure 22. Immiscibility between various BMP+ cation-based ILs (having different anions (BF4−, OTf−, and TFSI−)) and de-ionized water (DIW). (a) Photographs and (b) 19F NMR spectra.
To act as a water-repellent ion-conducting protective layer, the IL skinny gel allows the dissociation of Zn salts (Zn(TFSI)2), in addition to the immiscibility with water described above. In general, most hydrophobic solvents tend to show weak polarity, resulting in poor salt dissociation. The salt- dissociating capability of various hydrophobic solvents (hexane, toluene, chloroform, butyl acetate, and BMPTFSI) was investigated, all of which were not miscible with water (Figure 23a). Into each solvent, 0.3 m (mol kg−1) Zn(TFSI)2 was added. Only BMPTFSI was capable of dissolving Zn(TFSI)2, whereas white precipitates or a cloudy solution were formed in other solvents (Figure 23b). This salt dissociation of the BMPTFSI was confirmed by conducting vis-NIR spectroscopy analysis. The BMPTFSI showed no absorbance, unlike other solvents (Figure 23c). The feasibility of applying the Zn(TFSI)2-dissolved
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BMPTFSI electrolyte (hereinafter, denoted as “IE”) in Zn electrodes was explored (Figure 24). The IE showed stable Zn plating/stripping cyclability (>1000 h) with a high C.E. (>98.7%), demonstrating its electrochemical viability for redox reactions of Zn electrodes.
Figure 23. Salt-dissociating capability of various hydrophobic solvents (hexane, toluene, chloroform, butyl acetate, and BMPTFSI). (a) Photographs showing the immiscibility of the hydrophobic solvents with water.
(b) Photographs showing the salt (0.3 m Zn(TFSI)2)-dissociation in the hydrophobic solvents. (c) Vis-NIR spectra of the 0.3 m Zn(TFSI)2-containing hydrophobic solvents.
Figure 24. Electrochemical characteristics of the IE (0.3 m Zn(TFSI)2 in BMPTFSI). (a) Galvanostatic Zn plating/stripping cyclability of the Zn/Zn symmetric cell at a current density of 0.1 mA cm−2. (b) C.E. of the Ti/Zn asymmetric cell at a current density of 0.1 mA cm−2.
The immiscibility of the IE with an aqueous electrolyte (2 m ZnSO4, denoted as “AE”), commonly used to activate redox reactions of ZIBs cathodes, was investigated. It was clearly observed that the IE (bottom layer) was immiscible with the AE (top layer) (Figure 25a). This immiscibility between the IE and AE was verified by analyzing the Raman spectra (Figure 25b). The characteristic peaks assigned to TFSI− (742 cm−1)147 of the IE and SO42− (983 cm−1)123 of the AE were not found in the AE and IE phases of the mixture solution, exhibiting the immiscibility of the two phases. In addition, peaks of OH− (3234
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and 3412 cm−1) originating from water molecules were not detected in the IE phase of the mixture solution. Furthermore, the Raman spectra as a function of temperature (−15℃ and 45℃, Figure 25c) showed that the IE-AE immiscibility was maintained over a wide range of temperatures.
Figure 25. Immiscibility of the IL skinny gel and its physicochemical properties. (a) Photographs of the AE (2 m ZnSO4 aqueous electrolyte), IE (0.3 m Zn(TFSI)2 in BMPTFSI), and AE/IE mixtures showing the immiscibility. (b) Raman spectra (focusing on TFSI− and OH−) of the AE, IE, and AE/IE mixture. (c) Raman spectra (focusing on O−H stretching modes) and photographs (insets) of the AE/IE mixture at −15 and 45℃.
Figure 26. Physicochemical characteristics of the IL gel. (a) Photographs of the IL gel before (inset) and after UV curing-driven solidification. (b) Effect of the monomer content (0, 2, and 5 wt%) on the solidification of IL gels (after the UV-curing). (c) Changes in the characteristic FT-IR peaks assigned to the thiol (S−H) groups (2575 cm−1) and acrylic C=C bonds (1610–1625 cm−1) of the thiol-ene polymer network before/after UV-curing.
Figure 27. Photographs showing the AE-repelling behavior of the IL gel, in which the IL gel-coated glass fiber separator was inserted between a bare AE (2 m ZnSO4 aqueous electrolyte, bottom) and a dye (indigo carmine)-containing AE (top).
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Based on the physicochemical characterization of the IE, a self-standing IL gel was fabricated. The IE was mixed with the thiol-ene monomer mixture (TMPMP/TMPTA = 1/2 (mol/mol) was chosen to provide mechanical conformability and flexibility52) and then exposed to UV irradiation for 1 min, resulting in an IL gel (Figure 26a). The amount of the thiol-ene monomer mixture needed to enable the nonfluidic IL gel was found to be 5 wt% (Figure 26b). The UV curing of the thiol-ene polymer network was confirmed by monitoring the change in the FT-IR peaks assigned to thiol (S−H) groups (2575 cm−1)148 and acrylic C=C bonds (1610–1625 cm−1)149 before/after UV-irradiation-driven curing (Figure 26c). In addition, the gel content (i.e., insoluble polymer fraction after solvent (dimethyl carbonate followed by acetone) extraction150) of the resulting thiol-ene polymer network was higher than 99%, confirming the successful UV-irradiation-driven curing. Meanwhile, the water-repelling capability of the obtained IL gel was examined by conducting a model experiment (Figure 27). An IL-gel-coated glass fiber separator was prepared and inserted between a bare AE and a dye (indigo carmine)- containing AE. In contrast to a pristine glass fiber separator (chosen as a control sample), no change in the color was observed in the IL-gel-coated glass fiber separator, even after one month, demonstrating its viability in repelling the AE.
The IL gel precursor was spin-coated on top of a Zn foil, followed by UV-curing, yielding the IL- gel-skinned (thickness ~ 500 nm) Zn electrode (Figure 28a). The chemical elements ascribed to the IL and thiol-ene polymer skeleton of the IL gel are shown in Figure 28b. The optimal thickness of the IL skinny gel was determined by considering a trade-off between the cell overpotential and cycling stability of Zn/Zn symmetric cells. The EIS analysis of Zn/Zn symmetric cells showed that interfacial Rct between the AE and IL-gel-deposited Zn electrode tended to increase with the IL gel thickness (Figure 29a). The Rct value of 500-nm-thick IL gel was significantly low and showed no appreciable difference from that of 300-nm-thick IL gel. Meanwhile, the Zn plating/stripping cyclability was investigated as a function of IL gel thickness using the AE-containing Zn/Zn symmetric cells. The overpotential in the voltage profiles (Figure 29b) showed a similar tendency to the result of the aforementioned EIS analysis. However, the 300-nm-thick IL-gel-deposited Zn electrode presented an unstable voltage profile after 40 h and eventually lost its electrochemical activity (Figure 29c). This result reveals that 300-nm-thick IL gel is not structurally stable against the AE during repeated Zn plating/stripping. By considering the above-mentioned results, the optimal thickness of the IL gel was determined to be 500 nm. The stable Zn plating/stripping behavior obtained using the 500-nm-thick IL gel was further elucidated by analyzing surface/cross-sectional scanning electron microscopy (SEM) images of the Zn electrodes. The stable and uniform Zn plating/stripping occurred underneath the IL gel layer without impairing its structural integrity (Figure 30).
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Figure 28. (a) Cross-sectional SEM image of the ILG-Zn. (b) SEM and (c) EDS elemental mapping images of the ILG-Zn electrode.
Figure 29. Electrochemical analysis of the ILG-Zn electrodes as a function of IL gel thickness. (a) EIS profiles. (b), (c) Galvanostatic Zn plating/stripping profiles of the AE-containing Zn/Zn symmetric cells at a current density of 0.5 mA cm−2.
Figure 30. Cross-sectional SEM images showing the stable Zn plating/stripping underneath the IL skinny gel.
Prior to characterizing the electrochemical performance of the ILG-Zn electrodes, their chemical stability against water was investigated. The ILG-Zn and bare Zn electrodes were placed in the AE for one week and their structural change was monitored. Figure 31a shows that the bare Zn electrode (shown in the inset) was contaminated with randomly deposited passivation layers, which are known to arise from Zn(OH)2 byproducts.127,151 In comparison, the ILG-Zn electrode showed a smooth and clean surface. As additional evidence, the EIS profiles of AE-containing Zn/Zn symmetric cells were traced
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as a function of the elapsed time (Figure 31b and 31c). The growth of Rct was suppressed at the ILG- Zn electrode (~ 790 W cm−2 after 60 h), whereas the bare Zn electrode steadily increased with time (~
5240 W cm−2 after 60 h). To confirm this advantageous effect of the ILG-Zn electrode on the water tolerance, the Zn stripping capacities of the Zn/Zn symmetric cells were compared after they were stored for one week. Figure 31d shows that the ILG-Zn electrode maintained almost 93% of the initial capacity, which was higher than that (48%) of the bare Zn electrode. These results show that the IL skinny gel
can effectively suppress direct exposure of Zn electrodes to the AE while allowing the Zn2+ conduction.
Figure 31. Chemical/electrochemical stability of the ILG-Zn with AE. (a) SEM images showing the chemical stability of the ILG-Zn (vs. bare Zn (inset)) after being stored in the AE for one week. EIS profiles of the AE-containing Zn/Zn symmetric cells as a function of elapsed time; (b) Bare Zn and (c) ILG-Zn. (d) Galvanostatic Zn stripping capacities of the AE-containing Zn/Zn symmetric cells (ILG-Zn vs. bare Zn) after being stored for one week at a current density of 0.5 mA cm−2.
Figure 32. (a) C.E. of Zn plating/stripping on ILG-coated Ti (vs. bare Ti) at a galvanostatic capacity of 0.1 mAh cm−2 and a current density of 0.1 mA cm−2. (b) Voltage profiles (at 50th cycle) of the Zn plating/stripping cycling at a current density of 0.1 mA cm−2. Bare Ti vs. ILG-coated Ti.
The C.E. of Zn plating/stripping was investigated using Ti/Zn asymmetric cells,140,152 in which the IL skinny gel (thickness ~ 500 nm) was deposited on top of a Ti foil and the cells were cycled with a galvanostatic capacity of 0.1 mAh cm−2 at a current density of 0.1 mA cm−2. Figure 32a shows that the IL skinny gel remarkably improved the C.E. of Zn plating/stripping, which was highlighted by comparing the voltage profiles at the 50th cycle between the ILG-coated Ti and bare Ti (Figure 32b).
To further elucidate this advantageous effect of the IL skinny gel, in situ DEMS of Zn/Zn symmetric cells was conducted. Zn metals in contact with weak acidic aqueous electrolytes tend to provoke H2 gas evolution in the open-circuit voltage (OCV) stage through chemical side reactions.8,42 In addition,
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galvanostatic Zn plating leads to generation of electrochemically induced H2 gas along with consumption of electrons.140 Figure 33 shows that the H2 gas signal was barely detected at the ILG-Zn electrode compared with the bare Zn electrode, showing erratic profiles. The H2 gas evolution in aqueous ZIBs is known to cause serious problems, such as the decrease of Zn utilization efficiency, build-up of internal cell pressure, and water depletion.91 Therefore, the suppression of H2 evolution by the IL skinny gel is expected to play a vital role in achieving reliable electrochemical performance of Zn electrodes.
Figure 33. H2 gas signal of the ILG-Zn (vs. bare Zn) as a function of time, which was obtained by in-situ DEMS analysis of AE-containing Zn/Zn symmetric cells at a current density of 0.1 mA cm−2.
Figure 34. (a) Galvanostatic Zn plating/stripping cyclability of the AE-containing Zn/Zn symmetric cells (ILG-Zn vs. bare Zn) at an areal capacity of 0.1 mAh cm−2 and a current density of 0.1 mA cm−2. (b) Galvanostatic Zn plating/stripping cyclability of the Zn/Zn symmetric cells under a higher areal capacity and current density; 0.5 mAh cm−2 at 1 mA cm−2 and 1 mAh cm−2 at 2 mA cm−2. (c) Galvanostatic Zn plating/stripping cyclability of the (ILG-Zn/ILG-Zn) symmetric cell, (ILG-Zn/bare Zn) asymmetric cell, and (bare Zn/bare Zn) symmetric cell at a current density of 0.1 mA cm−2.
The reversible Zn plating/stripping behavior of the ILG-Zn electrode was investigated using Zn/Zn symmetric cells. Figure 34a shows that the bare Zn electrode failed to maintain its electrochemical cyclability (at a current density of 0.1 mA cm−2) after approximately 120 h, mainly because of the H2
gas evolution and accumulation of byproducts.153-154 By contrast, the ILG-Zn electrode presented stable and sustainable cycling performance over 1000 h, although a slightly higher polarization was observed
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during the initial cycles. This electrochemical reliability of the ILG-Zn electrode was also observed at faster current densities of 1.0 and 2.0 mA cm−2 (Figure 34b), indicating facile Zn2+ ion transport through the IL skinny gel. Meanwhile, to address the concern about the higher polarization of the ILG-Zn electrode, the Zn plating/stripping cyclability of an (ILG-Zn/bare Zn) asymmetric cell was examined.
The asymmetric cell showed a lower polarization than the (ILG-Zn/ILG-Zn) symmetric cell, which also appeared similar to that of the (bare Zn/bare Zn) symmetric cell (Figure 34c). This result indicates that the polarization of the (ILG-Zn/ILG-Zn) symmetric cell (shown in Figure 34a) may be caused by the double-thick ILG.
Figure 35. Galvanostatic Zn plating/stripping cyclability of the AE-containing Zn/Zn symmetric cells (ILG- Zn vs. bare Zn) at a higher areal capacity of 1.8 mAh cm−2 (corresponding to 90% DODZn) and current density of 0.5 mA cm−2.
Figure 36. Post-mortem analysis of the Zn electrodes after Zn plating/stripping cycling test. (a) SEM images (insets: photographs) of the bare Zn (top) and ILG-Zn (bottom), in which the Zn/Zn symmetric cells were cycled under an areal capacity of 0.1 mAh cm−2 and a current density of 0.1 mA cm−2. (b) XPS spectra (Zn 2p3/2 (left) and O 1s (right) peaks) of the bare Zn (top) and ILG-Zn (bottom). (c) TOF-SIMS mapping images (ZnO+ species) of the bare Zn (top) and ILG-Zn (bottom).
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The advantageous effect of the ILG-Zn electrode on the cycling retention was observed at a higher areal capacity of 1.8 mAh cm−2 (corresponding to 90% of DODZn) and a current density of 0.5 mA cm−2 (Figure 35). Under this harsh operating condition, the voltage profiles of the ILG-Zn electrode were stably maintained over 400 h, in comparison with the bare Zn electrode which lost its electrochemical activity after only 45 h.
To better understand the electrochemical performance of the ILG-Zn electrode, its structural change after the cycling test (50 cycles) was investigated. The bare Zn electrode was covered with randomly formed and pulverized byproducts (Figure 36a (top)). By comparison, a relatively flat and smooth surface was observed at the ILG-Zn electrode (Figure 36a (bottom)). This morphological result was consistent with those of the model study shown in Figure 31a.
Such structural stabilization of the ILG-Zn electrode was verified by conducting XPS analysis (Figure 36b). The bare Zn electrode showed a weaker intensity of the Zn 2p3/2 peak than the ILG-Zn electrode. Moreover, the Zn 2p3/2 peak position of the bare Zn electrode was shifted to higher binding energies, indicating the corrosion155 of Zn triggered by exposure to the AE. The deconvoluted O 1s peaks of the ILG-Zn electrode were observed at 530.3 and 531.9 eV, which were identical to those of a fresh Zn electrode. By comparison, the bare Zn electrode showed an upfield shift in the peak positions, which is attributed to chemisorbed oxygen originating from the formation of unwanted hydroxides and carbonates.156-157 The distribution of oxidized Zn species after the cycling test was investigated using TOF-SIMS. A small trace of ZnO+ ions was detected on the ILG-Zn surface, whereas the bare Zn electrode showed a high concentration of randomly distributed ZnO+ ions (Figure 36c).
Figure 37. LEIS area scan of the (a) bare Cu and (b) ILG-coated Cu after Zn electrodeposition with an areal capacity of 3 mAh cm−2.
Furthermore, LEIS158 was conducted to elucidate the ionic topology and resistance of the electrodeposited Zn surface. For this LEIS analysis, Zn with a capacity of 3 mAh cm−2 was electrodeposited on bare Cu and ILG-coated Cu, respectively. After the Zn deposition, the bare Cu showed a random and uneven ionic topology with high local resistance (2006 Ω cm−2) (Figure 37a). In comparison, uniform ionic topology with low local resistance (207 Ω cm−2) was observed at the ILG-
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coated Cu (Figure 37b). This structural stabilization by the IL skinny gel is ascribed to the suppression of direct contact of Zn with AE. In addition, the IL skinny gel can provide controllable nucleation sites for Zn2+ ions by an IL-driven electroshielding effect,51 thereby contributing to uniform Zn plating/stripping.
Figure 38. Electrochemical performance of the Zn/MnO2 full cells (containing aqueous electrolyte). (a) CV profiles (bare Zn vs. ILG-Zn) at a scan rate of 0.1 mV s−1. (b) Galvanostatic charge/discharge profiles (bare Zn vs. ILG-Zn) at a current density of 0.1 C.
Figure 39. EIS profiles of the Zn/MnO2 full cells (bare Zn vs. ILG-Zn) after being precycled at a current density of 0.1 C; (a) Bare Zn and (b) ILG-Zn. (c) GITT profiles of the Zn/MnO2 full cells (bare Zn vs. ILG- Zn) upon repeated current stimuli (at a current density of 0.1 C and interruption time between pulses of 1 h).
The electrochemical performances of Zn-ion full cells were examined, which were composed of Zn anodes, a-MnO2 cathodes (a-MnO2/carbon black powder/polyvinylidene fluoride = 70/20/10 (w/w/w)), and aqueous electrolyte (2 m ZnSO4 + 0.1 m MnSO4 in water). The cyclic voltammetry (CV) profiles (Figure 38a) showed no significant difference in the redox peaks between the bare Zn and ILG-Zn anodes. The ILG-Zn anode showed normal charge/discharge profiles at a current density of 0.1 C (1 C
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= 308 mA g−1 based on single-electron transfer between the Mn4+/Mn3+ redox pair)41, which appeared comparable to that of the bare Zn anode (Figure 38b). These results exhibit that the IL skinny gel does not impair the redox kinetics in the full cell.
Prior to examining the cycling performance of Zn/MnO2 full cells, the cells were precycled (two cycles at a current density of 0.1 C and rest for 24 h), and the change of their EIS profiles was monitored.
The Zn/MnO2 full cell with the bare Zn anode showed a large increase in cell resistance (Figure 39a), disclosing the accumulation of unwanted byproducts during the pre-cycling. The increase of Rct was pronounced, indicating the retarded charge diffusivity resulting from the electrode passivation. In contrast, the lower Rct was observed at the full cell with the ILG-Zn (Figure 39b). Such changes in the cell resistance during the precycling were verified by galvanostatic intermittent titration technique (GITT) analysis (Figure 39c). The rise of the cell polarization upon the repeated current was suppressed at the ILG-Zn anode compared with that at the bare Zn anode. The aforementioned results demonstrate that the IL skinny gel prevented the exposure of Zn anodes to AE, thus mitigating water-triggered interfacial parasitic reactions.
Figure 40. (a) Charge/discharge cycling performance (bare Zn vs. ILG-Zn) at a current density of 2 C. (b) Charge/discharge profiles (bare Zn vs. ILG-Zn) at the 1st and 600th cycles. (c) SEM images of the cycled ILG-Zn anode (inset: bare Zn anode).
The cycling performance of the Zn/MnO2 full cells was investigated at a faster current density of 2 C (Figure 40a). The capacities of the full cells tended to increase during the initial 150 cycles and gradually decreased with cycling, which is attributed to the gradual activation of MnO2 active materials.123,159 The full cell with the bare Zn anode showed capacity retention of 49.6% after 600 cycles.
By comparison, the ILG-Zn anode enabled the full cell to deliver high capacity retention of 95.7% up to 600 cycles, along with a high C.E. ~ 100%. Moreover, the full cell with the ILG-Zn anode maintained the two sloping voltage plateaus until 600 cycles (Figure 40b), indicating that the passivation of the Zn anode surface during the cycling could be alleviated. This superior cyclability was verified by the smooth and dense surface of the cycled ILG-Zn anode as compared with the result of the cycled bare Zn anode (Figure 40c), which appeared consistent with the results (Figure 40a) of the Zn/Zn symmetric