An Amphipathic Ionic Sieve Membrane for Durable and Dendrite-Free Zinc-Ion Batteries

Open AccessRenewablesRESEARCH ARTICLES20 Feb 2024An Amphipathic Ionic Sieve Membrane for Durable and Dendrite-Free Zinc-Ion Batteries Xian-Xiang Zeng, Shu Zhang, Tao Long, Qing-Yuan Zhao, Hong-Rui Wang, Wei Ling, Xiong-Wei Wu, Aiping Yu and Zhongwei Chen Xian-Xiang Zeng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Materials Science, Hunan Agricultural University, Changsha 410128 , Shu Zhang School of Chemistry and Materials Science, Hunan Agricultural University, Changsha 410128 , Tao Long School of Chemistry and Materials Science, Hunan Agricultural University, Changsha 410128 , Qing-Yuan Zhao School of Chemistry and Materials Science, Hunan Agricultural University, Changsha 410128 , Hong-Rui Wang School of Chemistry and Materials Science, Hunan Agricultural University, Changsha 410128 , Wei Ling School of Chemistry and Materials Science, Hunan Agricultural University, Changsha 410128 , Xiong-Wei Wu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Materials Science, Hunan Agricultural University, Changsha 410128 , Aiping Yu Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1 and Zhongwei Chen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] National Key Laboratory of Catalytic Energy Conversion, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023 https://doi.org/10.31635/renewables.024.202300045 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Rechargeable aqueous zinc-ion batteries are expected to be widely deployed for grind-scale energy storage due to the merits of low expenditure, safety, and so on. However, challenges on the Zn anode, including dendrite growth and parasitic reactions with aqueous electrolytes hinder its advancement. Hereby, an amphipathic ionic sieve (AIS) membrane was designed to screen Zn2+ from the bulk of aqueous electrolyte by ruling out water solvent and assisting Zn2+ to deposit with a smooth morphology under a confined shielding. These benign characteristics enabled AIS to run Zn symmetric cells over 2300 h with a voltage of 1 mA h·cm−2 at each cycle and tolerated long-term standby for more than 3 months. A low self-discharge rate and outstanding cycling stability were realized in V2O5/Zn batteries at equal weight for cathode and anode (N/P ratio ≈ 1.39). This asymmetrical wetting separator proved a facile strategy to solve interfacial dendrites and parasitic reactions in Zn metal-based batteries. Download figure Download PowerPoint Introduction Zinc, as a kind of nontoxic and low-cost metal, possesses high theoretical capacity (820 mA h·g−1 or 5851 mA h·cm−3) and low redox potential of Zn2+/Zn (−0.76 V vs standard hydrogen electrode), and has been utilized as the anode in primary battery for a long history.1–6 However, challenges such as the dendrite growth,7 parasitic reactions with water, no matter if it is alkaline, neutral, or acid,8–10 stand in the way of developing practical rechargeable zinc batteries.11,12 To deal with these issues, countermeasures mainly include the development of artificial solid electrolyte interphase (SEI),13–15 anode reconstruction,16–18 new liquid electrolytes with different additives,19–22 solvents,23,24 salts,25,26 and concentration,27,28 and solidified electrolytes.29–31 These strategies unveil a common denominator to prevent solvated water from direct or restricted contact with the Zn anode, which is beneficial to suppress water decomposition and Zn corrosion. However, these choices are more or less subjected to high-cost issues, poor contact, or unsatisfactory mechanical strength. As a frequently oblivious but essential component, the broadly used separators such as cellulose and glass fiber are hard to achieve the abovementioned functions in electrode and electrolyte, since they are either too thick (∼600 μm for glass fiber) or mechanically fragile during long-term operation.32–35 Additionally, these separators cannot suppress the parasitic reaction or dendrite growth in the aqueous electrolyte.33 The fundamental roots in the function of isolating water from the bulk electrolyte and only permitting Zn2+ to pass through for electroplating, minimizing the occurrence of parasitic reaction.36,37 Some polymer-based separators might enable selective Zn2+ permeation but encounter degradation in water or discounted desolvation and electrochemical kinetics.34,38 Therefore, the regulation of interface chemistry with thin and mechanically robust separators is of great importance.39,40 Strictly speaking, there has been almost no research focusing on separators with Zn2+ desolvation regulation or Zn2+ sieving capability in rechargeable zinc-ion batteries (ZIBs). Besides, the performance verification in the full batteries with fabricated separators is based on excessive Zn. Herein, we prepared a heterogeneous and thin composite polymer separator with an amphipathic ionic sievie (AIS) feature based on a hydrophobic microporous substrate. By elaborately grafting a copolymer of acrylic acid (AA) and acrylamide (AM) as the functional moiety, the surface wettability of the AIS substrate was engineered from near hydrophobicity to hydrophilicity, and thanks to the discrepant wettability, the zinc hexahydrate ([Zn(H2O)6]2+) underwent desolvation by preferentially binding between the water molecule and hydrophilic segment before its transfer to Zn surface for flat deposition, which also contributed to an augmented Zn2+ transference number in contrast to a fully hydrophilic separator, while the nearly hydrophobic surface of AIS restrained the continuous formation of passivating layer triggered by desolvated water molecules and possible water solvent from the bulk phase of an electrolyte. Under this context, the side reactions and Zn dendrite, after 3 months' standby have been greatly restrained, and Zn symmetric cells cycled with 2300 h at 1 mA h·cm−2 and V2O5/Zn batteries with a negative/positive capacity ratio of 1.39 (N/P ratio ≈ 1.39) were also demonstrated, which is one of the lowest value reported to date. Results and Discussion The amphipathic ionic sieve (AIS) was prepared through the dip coating method, during which the AM and AA monomers were initiated by ammonium persulfate to polymerize, and cladded on hydrophobic polytetrafluoroethylene (PTFE) porous substrate (Scheme 1a,b and Supporting Information Figure S1). Fourier transform infrared (FTIR) spectra showed typical peaks of N–H near 3400 ∼ 3200 cm−1, 2933 cm−1 for the C–H stretching vibration, strong peaks around 1655 and 1443 cm−1 of C=O in AM and AA monomers, and C–F peaks (1204 and 1144 cm−1) in PTFE (Scheme 1c). For comparison, a pure gel electrolyte membrane (Gel) was also prepared. Basic physics and chemistry characters for PTFE, AIS, and Gel are listed in Supporting Information Table S1. Notably, the thickness of AIS was merely about 20 μm (Figure 1a), much thinner than the dense and fragile dry gel electrolyte membrane (∼136 μm) ( Supporting Information Figure S2a,b). From the uniform element N and O mappings of the AIS surface (Figure 1b,c), the PTFE was covered successfully by a crosslinked network. Besides, it was feasible to scale up the AIS membrane by selecting a large-size PTFE substrate ( Supporting Information Figure S3). It is noteworthy that the AIS membrane showed much higher mechanical strength than the Gel as the electrolyte although its thickness was thinner compared with reported separators and solid-state electrolytes so far ( Supporting Information Figure S4 and Table S2), thereby proving its potential application prospect in ZIBs. Scheme 1 | (a) Reaction mechanism and formation process of the amphipathic membrane. (b) Scheme, illustrating the ion sieve process in the amphipathic membrane for Zn-ion batteries. (c) Fourier transform infrared (FTIR) spectra of polytetrafluoroethylene (PTFE) and the AIS membrane were obtained. Download figure Download PowerPoint More importantly, the AIS showed different wetting behaviors in contrast to PTFE and Gelas an electrolyte. We observed that the PTFE microporous membrane showed a contact angle of 130° with water (Figure 1d) while the dry gel electrolyte membrane showed a super-hydrophilic behavior on both sides ( Supporting Information Figure S2c,d). In contrast, the AIS showed asymmetric wettability, the contact angle at one side was ∼90° and the other was ∼45° (Figure 1e). The intensity and location change of the hydroxide group could be applied to indicate the interaction between the substrate and water molecule. Additional Raman tests for the AIS membrane and dry gel electrolyte were conducted to distinguish the discrepancy ( Supporting Information Figure S5). We found that the intensity of water molecules at the nearly hydrophobic side was lower than that of the hydrophilic side, and both intensities were lower than that of the dry gel electrolyte. The result was reasonable as the hydrophobic side, expelled water molecules from its surface and displayed lower intensity caused by a hydroxide group. Such amphipathic property catered for the purpose of Zn2+ migration from the bulk electrolyte to the Zn surface without compromising the reaction kinetic but constraining side reactions caused by free water molecules. Figure 1 | (a) SEM image of cross-section and (b) surface area for amphipathic membrane, and (c) corresponding nitrogen and oxygen element mapping. Contact angle test (up) and photograph (down) for (d) PTFE and (e) AIS membrane. SEM, scanning electron microscopy; PTFE, polytetrafluoroethylene; AIS, amphipathic ionic sieve. Download figure Download PowerPoint Thermal stability was a vital index for the separator. Four stages of weight loss appeared in the thermogravimetric curve of AIS ( Supporting Information Figure S6). The first step occurred below 120 °C, corresponding to the loss of water adsorbed, the second stage represented the decomposition of –NH– in AM from 200 to 300 °C, the third stage, which occurred from 370–480 °C was attributable to the overall degradation of the polyacrylamide/polyacrylamide (PAM/PAA) polymer backbone,41 and finally overlaps with PTFE substrate at 500 °C. These results indicate that AM and AA crosslinked on the PTFE substrate and altered the wetting ability. The validity of AIS on the electrochemical stability of electrolytes was further testified, which was long trapped by the electrolysis of H2O and restricted the application of aqueous ZIBs. Firstly, the AIS impacted the desolvation process of Zn2+ and further on Zn redox in aqueous electrolytes. The electrostatic potential (EP) was analyzed by density functional theory (DFT) calculations and used to compare the adsorption effect between [Zn(H2O)6]2+ and diverse substrates. As shown in Figure 2a, the charge distribution in PTFE is neutral, demonstrating its difficulty in absorbing [Zn(H2O)6]2+, and without interaction with water molecules. However, the [Zn(H2O)6]2+ could be absorbed by the copolymer of AM and AA as there existed a distinctly discrete area of positive and negative charges, and the binding energy of the copolymer with [Zn(H2O)6]2+ (−2.41 eV) was much larger than sole AM (−1.80 eV) and AA (−1.34 eV) (Figure 2b and Supporting Information Figures S7 and S8), revealing synergic bonding effects between AM and AA toward [Zn(H2O)6]2+. Figure 2 | (a) Electrostatic surface potential of PTFE and copolymer of acrylamide (AM) and acrylate acid (AA). (b) Binding energy of between with [Zn(H2O)6]2+ and AM, AA, and the copolymer of AM and AA. (c) Binding energy between H2O (up) and [Zn(H2O)6]2+ (down) with copolymer of AM and AA. (d) Electrochemical impedance spectra of Zn symmetric cell before and after polarization. (e) Activation of stainless steel symmetric cells at different temperatures. The atom sizes of AM and AA, scaled up for aesthetic reasons. AIS, amphipathic ionic sieve; PTFE, polytetrafluoroethylene. Download figure Download PowerPoint Further, to unravel the reaction mechanism, we investigated the binding energy to explore whether the introduced functional polymer on the PTFE substrate would participate in the desolvation process of [Zn(H2O)6]2+ (Figure 2c and Supporting Information Table S3). To our surprise, the copolymer of AM and AA initially combined with H2O surrounding Zn2+ with low binding energy (−15.23 eV), confirming that their copolymer improved the hydrophilicity of PTFE substrate and helped [Zn(H2O)6]2+ to desolvate, with the binding energy between H2O and Zn2+ (8.41 eV) being greater than 0, revealing that this reaction was nonspontaneous. Thus, the PTFE membrane modified by the copolymer was analogous to the molecular sieve to lock H2O but permitted the Zn2+ infiltration with an enhanced ion transference number (0.42) (Figure 2d), which was higher than the typical value of aqueous electrolyte (generally below 0.4),42,43 and the heterogeneous wettability in AIS was benefited to [Zn(H2O)6]2+ desolvation, verified by the reduced desolvation energy of hydrated zinc ion from 61.6 kJ mol−1 for the dry gel electrolyte to 52.3 kJ mol−1 for the AIS membrane ( Supporting Information Figure S9). Further, the activation energy of AIS (0.04 eV) was much lower than that of the Gel (0.1 eV) (Figure 2e and Supporting Information Figures S10 and S11). The above-mentioned merits of AIS were verified by Zn symmetric cells with 3 M ZnSO4 electrolyte. As shown in Figure 3a, the AIS maintained the polarization voltage within 150 mV after 2340 h. The polarization voltages were 45, 60, 70, 85, and 110 mV for AIS-based Zn symmetric cells from 0.1 to 1 mA cm−2, respectively (Figure 3b). By contrast, Zn symmetric cells assembled with PTFE could not work at the initial stage and the Gel-based cells failed after cycling for only about 300 h. It should be noted that the Gel electrolyte with comparable thickness to AIS was mechanically damageable and suffered from decomposition in water ( Supporting Information Figure S12). When the areal capacity was increased to 1 mA h cm−2, the polarization voltage was almost unchanged, even slightly lower than the original value, which was a common behavior in zinc ion batteries.11 Figure 3 | (a) Voltage-time profiles of Zn symmetric cells with AIS, Gel, and PTFE as separators. The inserts are curves after restarting for 135 days' stand-by (left) and voltage variation versus temperature (right), respectively. Magnified view of voltage-time curves at different current densities with AIS, Gel, and PTFE as the separator (a) at different current densities and (b) long-term cycling with a capacity of 1 mA h cm−2. AIS, amphipathic ionic sieve; PTFE, polytetrafluoroethylene. Download figure Download PowerPoint Wondrously, the Zn symmetric cells could restart and operate normally after resting for 135 days (the left inset in Figure 3a), and no short circuit was found after continuous operation for 2000 h. Meanwhile, the polarization voltage was gradually reduced from 90 to 50 mV when the temperature increased from 25 to 40 °C and gradually recovered after temperature restoration (the right inset in Figure 3a), clearly illustrating satisfactory temperature tolerance of the AIS. The polarization voltage after cycling for 2000 h was maintained at 100 mV (Figure 3c), indicating the long-term stability of AIS. However, Zn symmetric cells assembled with cellulose separator became short-circuited ( Supporting Information Figure S13). One possible reason is that due to the poor mechanical strength of the separator, it failed to withstand the dendrite piercing caused by uneven Zn deposition. The abovementioned results consistently indicated that the AIS membrane was involved in the solvation process and that the asymmetric wettability design reduced the occurrence of parasitic reactions and dendrite growth. Post-mortem of the cycled Zn anode with different separators were recorded. Other Zn anodes with cellulose at a comparable thickness and Gel are chosen as control. The Zn presents an irregular flower-like surface after 57-h operation compared with the pristine Zn anode (Figure 4a and Supporting Information Figure S14a,b), which easily pierced the separator and caused cell failure. While the Zn surface with the Gel electrolyte exhibited an uneven and irregular layered structure after 280 h, some of the radially vertical Zn flakes might have been caused by the short circuit failure of the battery (Figure 4b and Supporting Information Figure S14c). In stark contrast to the cellulose separator and Gel as an electrolyte, the Zn anode with the AIS separator showed a dense and flat plane without apparent dendrites and pores after cycling for 2340 h (Figure 4c and Supporting Information Figure S14d). Based on these Zn morphology and separator attributes, possible mechanisms for the three separator types were proposed (Figure 4d–f). The Zn2+ flux in the thin and porous cellulose separator was arbitrary and uncontrollable during plating while the Zn2+ was partly confined under the totally hydrophilic Gel as an electrolyte. Meanwhile, the AIS membrane showed a heterogeneous wetting ability to pre-sieve Zn2+ from the bulk of electrolyte with suppressed side reactions between the Zn anode and water molecule and exhibited spatially confined dendrite growth as well. Figure 4 | Scheme illustration of ion transfer process within (a) cellulose, (b) Gel as an electrolyte, and (c) AIS separator. SEM images of Zn anode after cycling for (d) 57 h with cellulose, (e) 280 h with Gel as an electrolyte, and (f) 2340 h with AIS separator. (g) Zn 2p, (h) O 1s, and (i) S 2p XPS spectra of Zn anode after cycling for 2340 h (up) and Zn metal stored in air (down) at the same time. AIS, amphipathic ionic sieve; SEM, scanning electron microscopy; XPS, X-ray photoelectron spectroscopy. Download figure Download PowerPoint Through the X-ray photoelectron spectroscopy (XPS) analysis, the main composition of the Zn anode with AIS separator was Zn4(OH)6SO4·xH2O after 2340-h operation,21,44,45 which was distinctively different compared with by-products such as Zn–O (∼529.8 eV) and O–H (. ∼531.7 eV) on the Zn anode stored in air for same time frame (Figure 4g–i). Under the protection of these passivation layers, the Zn/AIS/Cu cell showed an average Coulombic efficiency of 90%, which was generally observed in aqueous electrolytes without anode or electrolyte modification but with dramatic fluctuation, reaching as low as 20%;33 also, much longer lifespan and smaller overpotential (1500 h, 35 mV) were realized concurrently compared with Zn/Gel/Cu cells (500 h, 40 mV) under the sufficient Zn supply ( Supporting Information Figure S15), signifying that the AIS membrane could minimize the parasitic reaction of ZIBs and prolonged the lifespan at working and standby status. After activating with 300 mA g−1 for 3 cycles, the voltage of fully charged ZIB with an AIS separator dropped from 1.6 to 1.15 V, and the capacity retention rate reached 91.12%, corresponding to a self-discharge rate of 0.37% h−1 ( Supporting Information Figure S16). In comparison, Zn/Gel/Cu cells showed much lower capacity retention (70.83%) although with a similar voltage drop, and two folds self-discharge ratio (0.88% h–1), showing the superiority of AIS in stabilizing Zn anode. The electrochemical performance of Zn/V2O5 batteries with AIS membrane was further studied. Cyclic voltammetry tests were conducted at 0.1 mV s−1 from 0.2 to 1.6 V. Two pairs of redox peaks indicated that pentavalent vanadium underwent a two-electron redox process ( Supporting Information Figure S17). The potential difference of Zn/AIS/V2O5 was 168 and 120 mV, respectively, significantly lower than that of a Gel-based battery (255 and 318 mV). The Zn/AIS/V2O5 cell also showed more excellent rate performance (Figure 5a). The specific capacity attained 455 mA h g−1 at 0.3 A g−1, while the capacity was still maintained at 140 mA h g−1 at 10 A g−1 (Figure 5b). By contrast, the Zn/Gel/V2O5 battery merely released less than 100 mA h g−1 at 10 A g−1 ( Supporting Information Figure S18). Moreover, to check the AIS under a more realistic condition, the mass ratio of Zn anode and V2O5 was fixed at 1 (N/P = 1.39), which was one of the lowest N/P ratios; the assembled battery also showed both excellent rate and long-term cycling performances (Figure 5c and Supporting Information Figure S19), which also surpassed the performance of the Gel as an electrolyte, cellulose, and Whatman separators-based full batteries with a proportionable N/P ratio ( Supporting Information Figure S20). Figure 5 | (a) Rate performance comparison of Zn/V2O5 batteries with Gel as an electrolyte and AIS separator. (b) Charge–discharge curve of AIS-based Zn/V2O5 batteries, and (c) cycling stability comparison for Gel and AIS membrane-based ZIBs different excessive Zn amount. AIS, amphipathic ionic sieve; ZIBs, zinc-ion batteries. Download figure Download PowerPoint Conclusion An amphipathic membrane was prepared by impregnating hydrophobic substrate into the hydrophilic polymer with a facile method and realized the purpose of sieving zinc ions for dendrite-free plating from zinc hexahydrate via competitive bonding of water with hydrophilic moieties. Restraining parasitic reactions resulted from zinc anode and free water molecule in solvation sheath. Thanks to the asymmetric wettability, the amphipathic membrane assisted in constructing a sturdy interface protection layer that could tolerate temperature variation and operate stably after long-term standby. In addition, the improved capacity retention and rate performance of the full battery demonstrated the superiority of AIS in rechargeable ZIBs. This AIS might also enlighten a promising design guideline to solve the problem of dendrites and parasitic reactions in other metal batteries. Supporting Information Supporting Information is available and includes the scanning electron microscopy (SEM) images and photographs of PTFE, Gel as an electrolyte, AIS membrane, and zinc anode after cycling, mechanical tests, Raman spectra, thermogravimetric (TG) curves, calculation results for electrostatic potential (EP) and binding energy, EIS spectra and desolvation energy, electrochemical measurements for Coulombic efficiency, self-discharge, rate performance, and long-term cycling tests. Tables of basic physiochemical properties for separators and electrolytes, binding energies, and batteries' performance comparison with published reports are also incorporated. Acknowledgments This work was supported by the National Natural of the and of Hunan the Natural of Hunan the research of Department of Hunan and the Natural and of University of and Institute for Zhang Chen Yu Chen Chemistry for Rechargeable Zinc-Ion Wang Zhang Zhang Wang Rechargeable Based on The Rechargeable and on in Rechargeable Chen Batteries with by as a Wang Zhang with for Wang in Zeng the Zn and in Zhang and in Zinc-Ion A Zhao and Rechargeable Wang in Rechargeable Yu Chen of Rechargeable Energy Zhang Chen Wang Zeng Zhang Wu of Zn Chemistry for Chen and a for Zeng Zhang by for Wang Zhang Zhang by for Wang Zhao Wang Wu Zeng of Dendrite-Free with from Dendrite-Free for Zinc-Ion Energy Wang Zhang Wang Wang Zhao Zn2+ to for Zhao Wang Chen via Wang Zhao of for Zhang Wang Zn with for Wu Zeng Wang Wang Wu Rechargeable with and Energy Energy Wang Wang Zhang Zhang and Durable Wang Zhang for Rechargeable Wang Wang for Wang and with a Dendrite-Free in an Energy by Gel for Dendrite-Free Chen Chen and Dendrite-Free Zhang Zeng Wu Wang via Gel with Zn Chen for Rechargeable Yu Wei as toward Zn Zhang Zhang and to for a Zn and Zhang Wang Wang Membrane for Zinc-Ion Zhao Wu Chen Wang Wu Wang Wu that and for as a Ionic Sieve Membrane for Chen Zhao Wang Long Wu Wu Zeng as the for and Zhang and Through Chemistry for Dendrite-Free Under and of and in of and and of in and Zhang Wang Wang Wang Reaction for at 20 mA and 20 Wang for Long Wang Wu Chen the and for Information Chinese Chemical work was supported by the National Natural of the and of Hunan the Natural of Hunan the research of Department of Hunan and the Natural and of University of and Institute for