High-Rate Charging of Zinc Anodes Achieved by Tuning Hydration

Oct 5, 2016 - Department of Materials Science and Engineering, Kyoto University, ... Institute of Advanced Energy, Kyoto University, Kyoto 611-0011, J...
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High-Rate Charging of Zinc Anodes Achieved by Tuning Hydration Properties of Zinc Complexes in Water Confined within Nanopores Akira Koyama,† Kazuhiro Fukami,*,† Yuya Suzuki,† Atsushi Kitada,† Tetsuo Sakka,‡ Takeshi Abe,‡ Kuniaki Murase,† and Masahiro Kinoshita*,§ †

Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan Department of Energy and Hydrocarbon Chemistry, Kyoto University, Kyoto 615-8510, Japan § Institute of Advanced Energy, Kyoto University, Kyoto 611-0011, Japan ‡

ABSTRACT: Rechargeable batteries constructed with highenergy-density metal anodes, such as zinc and lithium, often suffer from dendrite formation during high-rate charging, which can lead to short circuits and reduced battery life. Here we report a novel method of realizing high-rate charging of zinc anodes together with the suppression of zinc dendrite formation by remarkably accelerating the electrodeposition within the nanopores of a porous electrode. By tuning not only the affinity between the wall-surface of nanopores and water but also that between the zinc complex with polyvalent carboxylates and water, we can establish a condition under which a surface-induced phase transition (SIFT) occurs. With the occurrence of SIFT, the penetration of zinc complexes into the nanopores becomes quite fast toward the formation of a second phase within the nanopore where the concentration of zinc complexes is orders of magnitude higher than in the bulk solution. More specifically, we use a hydrophobic nanoporous electrode, zinc complexes that behave as hydrophobic solutes, and nanopores with a sufficiently small diameter. Due to the fast penetration originating from SIFT, high-rate charging of zinc anodes and the suppression of dendrite formation are simultaneously achieved by continuous deposition of zinc without the depletion of zinc complexes in the nanopores. It is emphasized that nanometer-sized pores play a crucial role for the present technique for high-rate charging: Pores with a diameter of ∼3 nm induce SIFT, while mesopores several tens of nanometers in diameter do not. This behavior is in marked contrast with that exhibited in the absence of SIFT. Without SIFT, the supply of zinc complexes is made in accordance with the usual Fickian diffusion, zinc complexes are rapidly depleted within a nanopore, continuous high-rate charging is not possible, and matters become more serious as the nanopore diameter decreases.

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INTRODUCTION Rechargeable batteries with high energy density are required for further development of portable electronics and electric vehicles. Batteries constructed with elemental negativeelectrode (anode) materials, such as zinc and lithium, possess much higher energy densities than lithium ion batteries.1−4 Because charge and discharge of metal anodes correspond to electrodeposition and electrodissolution of metals, respectively, control of metal surfaces deposited and dissolved during the charge/discharge cycles is a crucial issue in achieving high durability and safety of metal anode batteries. In addition, highrate charging is definitely desired, but it results in a rough surface of metal deposited on and dissolved from the electrode. In the most serious case, dendritic metal is deposited under diffusion-limited electrodeposition (here, “diffusion” denotes the usual Fickian diffusion),5 and the growth causes short circuit of batteries.1 To date, many trials in the utilization of additives such as levelers and brighteners have been reported to achieve high-rate charging without suffering from the dendrite problem.6−12 However, such additives often decompose on the © XXXX American Chemical Society

electrode during high-rate charging. Even if the additives are effective for suppression of dendritic metal growth, maximum charging rate cannot be higher than the diffusion-limited rate of metal ions. Hence, both the occurrence of dendritic metal deposition and the limitation of charging rate by diffusion of metal ions restrict the charging rate of metal anodes. Thus, the practical use of metal anode batteries becomes factual once we overcome these two limitations. As porous materials are widely used for energy devices, researches on liquid state and ion dynamics in the pores have been advanced to improve their performance.13−19 Recent studies have revealed that liquid states in confined nanoscale pores differ markedly from those in bulk solutions.17−23 In the nanoporous environment, most of the molecules within the liquid interact with the surrounding pore-wall surface, and therefore, the liquid structure and dynamics are highly Received: July 13, 2016 Revised: September 30, 2016

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DOI: 10.1021/acs.jpcc.6b07030 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. (a) Current density vs potential curves, measured on flat silicon electrode at a scan rate of 10 mV·s−1 in malonate solution, pH 5.0. (b) Temporal profiles of the potential of flat and nanoporous silicon in malonate solution, pH 5.0. (c, d) Surface views of (c) flat silicon and (d) porous silicon after electrodeposition of zinc. (Insets) Magnified views of the electrode surface. (e) Cross-sectional SEM images of porous silicon after electrodeposition of zinc. Current densities for electrodeposition were unified to −10 mA·cm−2. Scale bar is 3 μm for larger images and 1 μm for insets.

even when they are rapidly consumed within the nanopores due to high-rate charging. It is therefore expected that the deposition of metallic zinc proceeds within the nanopores precedent to the top surface of the porous layer. Moreover, the depletion of zinc precursors, which is usually inevitable at the interface of a flat electrode, may be prevented within the nanopores, resulting in the suppression of zinc dendrites. In the present study, we experimentally demonstrate these fascinating features for the first time. Care must be taken, however, to establish conditions under which SIFT is induced: First, a porous silicon electrode with an average pore size of ∼3 nm (referred to as nanopores hereafter) and hydrophobic pore-wall surfaces is utilized; and second, zinc precursors are made sufficiently hydrophobic by systematically tuning their charge state by use of polyvalent carboxylate anions as ligands of zinc cations. With these contrivances, SIFT is successfully achieved, leading to preferential promotion of electrodeposition of metallic zinc within nanopores. Notably, the morphology of zinc deposits is determined by the shape of nanopores, resulting in suppression of zinc dendrites. High-rate charging is realized by the continuous filling of nanopores with zinc metal.

influenced by the properties of the surface. As a striking example, Kinoshita24,25 proposed the concept of a surfaceinduced phase transition (SIFT) on the basis of integral equation theory (a statistical-mechanical theory) for confined liquids. The concept was corroborated by a computer simulation study.26 SIFT is most likely to occur for water containing a low concentration of hydrophobic solutes confined by a hydrophobic surface or for nonpolar liquid containing a low concentration of hydrophilic solutes confined by a hydrophilic surface. For instance, when the concentration of hydrophobic solutes in bulk solution is gradually increased while the diameter of hydrophobic pores is kept constant, at a threshold concentration the pore space is filled with a second phase where the solute concentration is orders of magnitude higher than in the bulk. This is the so-called SIFT. When the pore diameter is gradually decreased while the solute concentration in bulk solution is kept constant, at a threshold diameter the same transition takes place. We have demonstrated the concept in the electrochemical deposition of platinum within a nanoporous silicon electrode with hydrophobic pore-wall surface, using sufficiently large platinum complex anions that behave like hydrophobic solutes.20−23 When SIFT occurs, strongly attractive long-range interactions come into play between the pore-wall surface and platinum complex anions toward formation of the second phase within the nanopores. These interactions contribute to the fast penetration of platinum complex anions into the nanopores and drastically increase the apparent mass-transfer rate from bulk solution to the pores,20 and the resulting accumulation of platinum complex anions causes the dense deposition of metallic platinum in the pores.23 The deposition rate required for metallic zinc considered in the present study is much higher than that actually occurring for platinum20−23 mentioned above. Theoretically, under the conditions of SIFT, fast supply of zinc precursors into the nanopores can be realized and the depletion does not occur

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EXPERIMENTAL SECTION Fabrication of Porous Silicon. Porous silicon was prepared by the anodization of n-type silicon (100) in 22 wt % HF solution (48 wt % HF/ethanol = 1:1.7 in volume). The average pore size of the material was tuned by controlling the silicon wafer resistivity and current density to 10−20 Ω·cm and 2.0 mA·cm−2, respectively, for ∼3 nm pores (nanopores) and to 0.02 Ω·cm and 80 mA·cm−2, respectively, for ∼20 nm pores (mesopores). The prepared silicon electrode was illuminated during the anodization for hole generation. The duration of anodization was tuned to make the thickness of porous layer ∼1.5 μm for nanoporous electrode and ∼4.0 μm for mesoporous electrode. The anodized area was 0.79 cm2. The porosities of as-anodized porous silicons were determined B

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not rapidly depleted compared to the surface of the flat silicon electrode. Under diffusion-limited conditions at such low potential, the dendritic growth of zinc preferentially proceeds on the electrode surface, which is indicated by the spontaneous potential fluctuation that was observed as the potential shifted into the negative range.5 The formation of dendrites is also consistent with SEM observations of the electrode surfaces. Zinc dendrites are detected on the surface of flat silicon after 20 s of electrodeposition (Figure 1c), whereas no dendrites are observed on the porous silicon surface at 20 and 40 s (Figure 1d). These times for flat and porous electrodes correspond to durations of high potential. Cross-sectional SEM imaging reveals that zinc deposition occurs within the porous layer (Figure 1e). As zinc deposition is not accompanied by hydrogen evolution at high potential, most of the electric current would be consumed by zinc deposition within the nanopores. Zinc dendrites are formed on the surface of the porous silicon layer after 60 s of electrodeposition (Figure 1d), indicating that the vacant pore volume is markedly decreased (Figure 1e), thereby promoting zinc deposition on the top surface. Taken together, these findings demonstrate that zinc dendritic growth is delayed by 40 s when flat silicon is replaced by nanoporous silicon. The decrease in effective current density below the diffusionlimited current density, due to enlargement of the specific surface area of the porous electrode, is generally effective for suppressing dendrite formation. However, the entire pore wall must serve as a reaction site for this process to proceed efficiently. Pore openings are generally advantageous as reaction sites compared to internal pore sites, regardless of the reaction rate,27,28 as reactants must be supplied from the surrounding bulk solution and because the transfer of solutes proceeds slowly, particularly in nanoporous media with high porosity.1,29 Metal deposition reactions therefore occur preferentially at pore openings, leading to the rapid closing of pores, which would contain vacant spaces.28 As a consequence, simple enlargement of the specific surface area is not considered to be an essential factor for the long-term suppression of zinc dendrite formation or the filling of pores. Because dendritic growth results from the steep concentration gradient of zinc precursors at sites of protuberance, a more uniform concentration profile of zinc precursors is crucial for the additive-free suppression of dendrite formation. Thus, avoiding the depletion of zinc precursors at the electrode interface is necessary for the suppression of dendritic growth, as is demonstrated in Figure 1. From an electrochemical viewpoint, the prevention of such depletion, even at a high charging rate (high current density), should be achieved when the concentration of zinc precursors is kept sufficiently high at the electrode surface and the apparent mass-transfer rate becomes much greater than in cases of Fickian diffusion. These conditions can be simultaneously satisfied when the concept of SIFT is applied. We next experimentally verified whether the two requirements, namely, low-charge state of the zinc precursors and small pore size, were achieved in this system. Advantage of Lower Charge State of Zinc Complexes for Surface-Induced Phase Transition. The solution pH was tuned to control the amount of zinc deposits in the nanopores without changing the bulk concentration of zinc and malonate species in the electrolyte. Aqueous malonate solution was selected as the electrolyte because the utilization of polyvalent carboxylate anions as ligands enables the charge state of zinc complexes to be controlled by pH tuning. Equilibrium

gravimetrically to be 76% for the nanoporous electrode and 33% for the mesoporous electrode. The samples were weighed three times: before the etching process, after anodization, and after removal of the porous layer in 25 wt % tetramethylammonium hydroxide solution. Electrodeposition of Zinc. As-anodized porous silicon was soaked in ultrapure water for 10 min to extract residual fluorides. The porous silicon layer was then washed thoroughly with water. The sample was not allowed to dry during the washing step, as the aqueous electrolyte solution for zinc deposition is not able to infiltrate pores after drying. After washing, zinc electrodeposition was performed under constant current density in two types of deposition baths. The first bath contained an aqueous solution of 0.1 M ZnSO4 and 0.1 M disodium malonate with pH values ranging from 3.0 to 5.0. The pH values were adjusted with sulfuric acid. The second bath contained an aqueous solution of 0.1 M ZnSO4 and 0.3 M citrate species (citric acid and trisodium citrate) with pH values ranging from 1.8 to 4.8. The pH values were adjusted by modifying the ratio of citric acid to trisodium citrate. The electrochemical cell consisted of porous silicon, a platinum rod, and Ag/AgCl saturated KCl as the working electrode, counter electrode, and reference electrode, respectively. Electrochemical measurements were performed on a potentio/galvanostat (Parstat 2273, Princeton Applied Research). Cross-sectional and surface views of the porous silicon electrode after zinc deposition were observed with a field emission-type scanning electron microscope (SEM; JSM-6500F, JEOL) and analyzed by energy-dispersive X-ray spectroscopy (EDS) in the SEM. Microstructures of the zinc deposited within the porous silicon layer were observed with a scanning transmission microscope (STEM; JEM-2100F, JEOL) equipped with a field emission gun operated at 200 kV. Chemical compositions were analyzed by EDS in the STEM. Specimens for STEM observations were prepared by slicing the porous layer into thin films by use of a focused ion beam (FIB; SMI9200; SII).

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RESULTS AND DISCUSSION Suppression of Zinc Dendritic Growth. We examined the capability of nanoporous silicon-based electrodes to promote zinc electrodeposition within the pores. The deposition behavior of zinc in an aqueous malonate solution of pH 5.0 was investigated by use of flat silicon and nanoporous silicon with an average pore size of ∼3 nm. Electrodeposition was performed at a current density of −10 mA·cm−2, which is similar to or a little larger than the diffusion-limited current density of −9 mA·cm−2 (Figure 1a). Current density was calculated from the projected area of the porous layer. Temporal profiles of the potential during electrodeposition of zinc for both flat and nanoporous silicon electrodes are compared in Figure 1b. The potentials of both electrodes remain high immediately after the onset of electrodeposition but subsequently shift to more negative values after ∼20 s. However, the potential of the nanoporous silicon electrode remains high for ∼60 s, whereas the flat silicon reaches low potential after only ∼20 s of electrodeposition. In general, during electrodeposition at diffusion-limited current density, zinc precursors are immediately depleted at the electrode surface. The resulting decrease in current density is compensated by the hydrogen evolution reaction, which causes a negative potential shift under diffusion-limited conditions. Therefore, the longer duration of high potential for the nanoporous silicon electrode suggests that zinc precursors are C

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The Journal of Physical Chemistry C equations between zinc cations and anionic ligands for zinc complexes dissolved in malonate solution are shown in Figure 2a. In Figure 2b, the expected amount of zinc complexes as a

Figure 3. Typical cross-sectional STEM images of nanoporous silicon after electrodeposition of zinc at a current density of −10 mA·cm−2 for 120 s in malonate solutions of pH (a) 3.0, (b) 4.0, and (c) 5.0. All the images are focused on the borders between the porous region and bulk silicon at the bottom of pores. The bright part of the image indicates a densely deposited region of zinc. Images on the right are magnified views of images on the left. Inset in panel b is focused on the dark part of the porous region. Zn/Si atomic ratios at designated points in the right images are confirmed by EDS point analysis to be 0.08, 7.53, and 7.10 for pH 3.0, 4.0, and 5.0, respectively. Scale bars in the left, right, and inset images are 100, 50, and 10 nm, respectively.

Figure 2. (a) Equilibrium between zinc cations and anionic ligands in malonate solutions. (b) Distribution of zinc complex species as a function of pH in an aqueous solution of 0.1 M malonic acid. L2− indicates malonate anions with the lowest valence. (c−e) Crosssectional SEM images of nanoporous silicon after the electrodeposition of zinc at a current density of −10 mA·cm−2 for 120 s in malonate solutions of pH (c) 3.0, (d) 4.0, and (e) 5.0. Scale bar is 1 μm.

function of pH is estimated from the formation constants taken from ref 30. Because the deprotonation of carboxyl groups does not occur at low pH, Zn2+ and ZnSO4 act as the main reactants. In contrast, the proportion of zinc/malonate complexes, particularly the zerovalent zinc complex (ZnL in Figure 2a,b), in the solution becomes larger with increasing pH and reaches a maximum above pH 6.0. Zinc electrodeposition at a constant current density was performed on the nanoporous silicon electrode for 120 s in malonate solutions of various pH values. The pH values range from 3.0 to 5.0 because the porous silicon layer chemically dissolves at higher pH. Cross-sectional SEM images of the porous silicon layer after electrodeposition in the malonate solutions are shown in Figure 2c−e. Zinc is deposited within the porous layer in all electrolyte solutions, but the density and morphology of zinc deposits varied markedly with pH. Within the porous layer, zinc was more densely deposited for the solution at pH 5.0 than for solutions at other pH values. Effective utilization of pore space for zinc deposition in the solution of high pH values was clarified by STEM observation and EDS point analysis. Figure 3 shows typical STEM images of the bottom of silicon nanopores after electrodeposition in malonate solutions of various pH values. Porous regions are composed mainly of silicon pore walls and zinc deposits. The bright part of the image indicates a densely deposited region of zinc. Zn/Si atomic ratios at designated points in Figure 3 are 0.08, 7.53, and 7.10 for pH 3.0, 4.0, and 5.0, respectively. This result indicates that zinc species are sufficiently supplied in pH

4.0 and 5.0 solutions, leading to densely deposited zinc even at the bottom of pores. However, a sparse region of zinc deposits is also notable in pH 4.0 solution, unlike that at pH 5.0. The inset in Figure 3b shows the dark part of porous region where isolated zinc particles are dispersed within the nanopores. Particle size is around 3−5 nm, which corresponds to the pore size. Thus, a certain proportion of pore space is not completely filled with zinc deposits in the solution at pH 4.0. This difference of filling rate leads to the lesser amount of zinc deposits within the nanoporous layer at pH 4.0. Deposition/dissolution tests in malonate solutions at pH 4.0 and 5.0, with applied prescribed cutoff potentials, were conducted in order to measure the specific amount of zinc within the nanoporous layer (Figure 4). The cutoff potentials were chosen so as to mainly allow zinc deposition or dissolution in the pores without other side reactions such as hydrogen evolution or silicon oxidation as well as dendritic growth of zinc. Since it was confirmed by our voltammetric experiment that silicon was oxidized above ∼−0.2 V (vs Ag| AgCl saturated KCl), the cutoff potential for zinc dissolution was set to −0.8 V. The current densities for deposition and dissolution were −10.75 and +1.075 mA·cm−2, respectively. On the flat silicon electrode, the amount of zinc deposits shows no difference between the solutions at pH 4.0 and 5.0, and they are almost completely dissolved, resulting in 94% anode dissolution efficiency. On the nanoporous silicon electrode, however, the deposition reaction proceeds much longer at pH 5.0 than at pH D

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Figure 4. Potential profiles of zinc deposition/dissolution process on flat and nanoporous silicon electrodes in malonate solutions of pH 4.0 and 5.0. Current densities for deposition and dissolution were −10.75 and +1.075 mA·cm−2, respectively.

4.0. Even when the slightly lower anode dissolution efficiency is considered, the larger amount of zinc deposits within the nanoporous layer in the solution of pH 5.0 is definite. A correlation between the amount of zinc in the nanopores and the zerovalent zinc complex (ZnL) is thus confirmed. Notably, a small amount of zinc deposits, including dendrites on the porous layer, was observed in the solution at pH 3.0. Three possible reasons are considered: First, chemical dissolution by acid attack in the solution at lower pH may occur after deposition. Second, the free malonates in the solution play a role of leveler (additive) like citrate, because the lower the pH, the more free malonates are in solution. Finally, a current density of −10 mA·cm−2 does not surpass the diffusion-limited current density only in pH 3.0 solution (please see Figure 6a). On the other hand, dendrites observed on the porous layer at pH 4.0 and 5.0 (Figure 2d,e) were formed after the pores have been filled with zinc. However, the dendrites are completely suppressed “during” the pore filling (Figure 1d,e). To confirm the importance of zerovalent zinc complexes for dense deposition of metallic zinc within silicon nanopores, electrodeposition was also performed in aqueous solutions with citrate as a ligand. In this solution, the proportion of zerovalent zinc/citrate complex [Zn(HL) in Figure 5a,b] is maximal near pH 3.0. Cross-sectional SEM imaging of the porous silicon layer in citrate solutions with pH ranging from 1.8 to 4.8 reveals that the density of zinc deposits within the porous layer is highest at pH 3.0, followed by pH 1.8 and 4.8 (Figure 5c−e). Morphological examination of the deposits revealed that, in contrast to malonate solutions, no dendritic growth occurred on the top surface of the porous layer in any of the citrate solutions. This difference was likely because free citric acids in the electrolytes greatly exceeded the amount of zinc species and functioned as an additive for leveling.6 However, the differences in the amount of zinc deposits within the nanopores cannot be explained by the leveling effect. The findings for the two polyvalent carboxylic solutions indicate that dense deposition of zinc within porous silicon requires that zerovalent zinc species are the main solutes in the electrolytes. This correlation cannot be explained by current efficiency for the electrodeposition of zinc. Current density versus potential curves measured in malonate or citrate solutions of various pH values show that the higher the pH value, the smaller the diffusion-limited current density (Figure 6). Therefore, the current efficiency for zinc deposition should be an increasing function of pH. However, the small amount of zinc within the porous layer, which is observed when malonate solutions of pH 3.0 or 4.0 and citrate solutions of pH 1.8 or 4.8

Figure 5. (a) Equilibrium between zinc cations and anionic ligands in citrate solutions. (b) Distribution of zinc complex species as a function of pH in an aqueous solution of 0.3 M citric acid. L3− indicates citrate anions with the lowest valence. (c−e) Cross-sectional SEM images of nanoporous silicon after electrodeposition of zinc at a current density of −10 mA·cm−2 for 120 s in citrate solutions of pH (c) 1.8, (d) 3.0, and (e) 4.8. Scale bar is 1 μm.

Figure 6. Current density vs potential curves, measured on flat silicon electrodes at a scan rate of 10 mV·s−1 in (a) malonate and (b) citrate solutions. (a) The pH values of malonate solutions are 3.0 (black), 4.0 (red), and 5.0 (blue). (b) The pH values of citrate solutions are 1.8 (black), 3.0 (red), and 4.8 (blue).

are used, contradicts their current efficiencies. On the other hand, the SIFT that occurs during metal electrodeposition within nanoporous electrode does explain the deposition behavior in the nanopores.21,22 The enrichment of reactants E

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The Journal of Physical Chemistry C resulting from SIFT is known to promote dense metal deposition within porous silicon electrodes, because large metal ions that are weakly hydrated also behave as rather hydrophobic solutes.21,22 Although the zerovalent zinc complexes are predicted to be slightly polarized, these complexes exhibit weaker hydration properties than the ionic species. Therefore, in electrolyte solutions containing a large amount of zerovalent zinc species, the occurrence of SIFT results in dense zinc deposition within the porous layer of silicon. In actual applications to electroplating and energy storage, where the concentrations of zinc species are much higher, a high ratio of zerovalent zinc complexes is also realized by adequately tuning the amount of polycarboxylate in the solution. The solute with high charge density is also difficult to enrich in hydrophobic nanopores, even when the bulk concentration is higher. Therefore, the relationship between the ratio of zerovalent complexes and the suppression of dendritic growth still holds in an electrolyte solution with a high zinc precursor concentration. Advantage of Smaller Pores for Surface-Induced Phase Transition. The mass-transfer rate of solutes through an aqueous solution within nanopores is generally very low, due to the complicated three-dimensional network of porous structures, and thus only a limited part of the total surface area tends to contribute to electrochemical reactions.27,28,31,32 For nanopores to function as a reaction field instead of the top surface of a porous layer, the deposition reaction must proceed inside the nanopores, which is normally unfavorable if the mass transfer of solutes into nanometer-scale spaces occurs by Fickian diffusion in accordance with the concentration gradient. Although the findings from the present study clearly show that zinc deposition can proceed within nanopores with an average diameter of ∼3 nm, the supply of zinc complexes into the nanopores is not made by Fickian diffusion. When porous silicon with an average diameter of ∼20 nm (referred to as mesopores hereafter) is used, only a few zinc deposits are observed within the mesopores at a current density of −10 mA· cm−2, which is larger than the diffusion-limited current density, and dendritic growth of zinc proceeds on the porous layer surface (Figure 7a,b). Under these conditions, limited deposition of zinc in mesoporous silicon is also observed in malonate solution at pH 5.0. Notably, low current density (−0.1 mA·cm−2) also prevents zinc deposition within the mesopores (Figure 7c,d). At a current density of −10 mA·cm−2, the potential of mesoporous silicon exhibits a negative shift at nearly the same time as flat silicon (Figure 7e). In this case, zinc complexes on the outside of the mesopores are rapidly depleted, a response that is consistent with the limited number of zinc deposits observed within the mesopores. At a current density of −0.1 mA·cm−2, the deposition reaction proceeds under electron-transfer-limited conditions, and the potential thus remains high. In this case, the amount of zinc complexes near the electrode surface is sufficient for zinc deposition under electron-transfer-limited conditions, and no dendritic growth occurs on the porous layer (Figure 7c). The results of EDS of the cross section of porous layer also indicate incomplete filling of mesopores with zinc (Figure 8). Zinc is hardly deposited at the bottom of the porous layer compared to the top surface. The finding that zinc was not deposited within the mesopores strongly indicates that the mass transfer of zinc complexes by Fickian diffusion is a slow process, even in mesopores, and demonstrates that zinc dendritic growth cannot be suppressed simply by use of porous electrodes. In addition to the

Figure 7. (a−d) Cross-sectional SEM images of mesoporous silicon after electrodeposition of zinc at current densities of (a, b) −10 mA· cm−2 for 120 s and (c, d) −0.1 mA·cm−2 for 12 000 s. Images in panels b and d are magnified views of the porous layers in panels a and c, respectively. Scale bars are (a, c) 1 μm and (b, d) 100 nm. (e) Temporal changes in the potential of flat silicon (black line) and mesoporous silicon (red lines) in malonate solution, pH 5.0. Current densities are −10 mA·cm−2 (solid lines) and −0.1 mA·cm−2 (broken lines).

hydrophobicity of pore walls and solutes, nanopore size is critical for promoting SIFT, as pores with a diameter of several tens of nanometers in size are too large to satisfy the conditions required for SIFT to occur.20 The present results suggest that the threshold diameter for SIFT is at least below ∼20 nm and should be around ∼3 nm, which is consistent with findings from previous studies.20−22 Without SIFT, the fast supply of metal complexes into mesopores is not possible. Role of Surface-Induced Phase Transition in Dendrite Suppression. Figure 9 illustrates how SIFT in nanopores enables dendrite suppression. When SIFT occurs, strongly attractive, long-range solute−surface interactions come into play, so that the second phase mentioned above can be formed with the space confined by the surface (Figure 9a).25 These interactions result in the fast supply of zinc complexes into nanopores and restrict the main reaction sites to the pore walls, rather than the top surface of porous layer, and thereby prevent dendritic growth outside the pores and promote continuous deposition within the pores (Figure 9c). The inset TEM image in Figure 3b clearly shows particulate zinc in the nanopores, indicating that there is no preferential growth of a specific crystal plane due to surface reorganization after the zinc complexes are reduced to metallic atoms even in the highconcentration environment of nanopores. Hence, the growth of nanoparticles and dense deposition of zinc resulting from the enrichment of zinc complexes within the pores leads to the complete filling of pore spaces (Figure 9e). If the pore-wall F

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Figure 8. (a, b) Cross-sectional views of (a) top surface and (b) bottom of mesoporous silicon after electrodeposition of zinc, with designated points where EDS analysis was conducted. Electrodeposition was performed in aqueous malonate solution, pH 5.0. under −10 mA·cm−2 for 120 s. The numbers at the spots in the images correspond to those in the following EDS analyses. Scale bar is 1 μm. (c) EDS spectra of Zn Kα, Zn Kβ, Zn Lα, and Si Kα for mesoporous silicon with zinc deposits.

Therefore, the combination of a hydrophobic nanoporous electrode and zerovalent zinc complexes induces SIFT and promotes electrodeposition within the nanopores, which is difficult to achieve by conventional electrochemical approaches.

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CONCLUSIONS

The findings from this study demonstrate that the promotion of SIFT in nanoporous electrodes enables high-rate charging of zinc anodes due to the orders of magnitude higher local concentration of zinc complex and their fast apparent mass transfer into pores. The suppression of zinc dendrite formation during high-rate charging is successfully achieved in an additivefree manner. The approach used here for promoting SIFT differs markedly from the strategies for realization of much higher charging rate as well as for suppression of dendritic deposition reported to date. Changes in volume and morphology of zinc deposits formed during charge/discharge (deposition/dissolution) cycles often cause serious problems for metal anode systems. However, these problems are potentially solvable by zinc deposition within porous electrodes. Unfortunately, the use of acidic or neutral zinc electrolyte as shown in the present study is not suitable for zinc−air batteries for two reasons: First, the compounds of transition metal for cathodes of O2 reaction are not durable in acidic solution. Second, highly concentrated Zn(II) is prepared in alkaline solution. However, the same concept is applicable to lithium metal anodes in nonaqueous solutions. In such systems, tuning the solvation properties of lithium cations and porous electrodes is a promising approach for suppressing dendrite formation by SIFT. For instance, utilization of a nonpolar solvent with high charge density lithium cations and a highly charged (hydrophilic) nanoporous electrode is expected to induce SIFT. However, because the use of porous electrodes that do not play a role of anode active materials lowers the energy density of batteries per volume or per weight, further

Figure 9. Illustration of zinc deposition processes (a, c, e) with and (b, d, f) without SIFT.

surface remains hydrophobic after discharging, the reproducibility of pore filling and dendrite suppression is highly expected. In contrast, zinc electrodeposition preferentially proceeds on the top surface of the porous electrode if the conditions do not satisfy the requirements for SIFT (Figure 9b,d). In such a case, supply of zinc precursor is expected toward the growing front of zinc dendrites (Figure 9d,f). Consistent with this finding, when the charge state of zinc cations was not tuned by polyvalent carboxylic species, the duration of suppression of dendritic growth became shorter.33 G

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The Journal of Physical Chemistry C

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development of such anode systems requires highly porous and sufficiently conductive nanoporous electrodes. Solvation properties of solutes in the solvent confined by surfaces on the scale of a nanometer are substantially different from those in bulk solvent. The present technique for high-rate charging of metal anodes is based on the concept of solvation property that is totally different from those ever reported. We are confident that the strategy reported here will contribute to a paradigm shift for the use of metal anodes in rechargeable battery systems.

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AUTHOR INFORMATION

Corresponding Authors

*(K.F.) E-mail [email protected]; phone +81 75 753 5477. *(M.K.) E-mail [email protected]; phone +81 774 38 3503. Author Contributions

A.Koyama and K.F. designed the experiments. Y.S. performed most of the experiments. A.Kitada, T.S., T.A., K.M., and M.K. analyzed the electrochemical data. A.Koyama and K.F. wrote the first draft of the paper. All authors contributed to the preparation of the final manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Yukiya Kato for reading through this paper and Kenji Kazumi for STEM observations. This work was supported by JSPS Grants-in-Aid for Scientific Research (B) (15H03877, to K.F.), and (A) (16H02411, to K.M.), by the Joint Usage/ Research Program on Zero-Emission Energy Research, Institute of Advanced Energy, Kyoto University (ZE27A-25), by a Grant-in-Aid for JSPS fellows (15J07665, to A.Koyama), and by the Core Research for Evolutional Science and Technology (CREST) program of JST (T.A.).

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b07030 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.jpcc.6b07030 J. Phys. Chem. C XXXX, XXX, XXX−XXX