Regulating the solvation sheath of zinc ions by supramolecular coordination chemistry toward ultrastable zinc anodes

Aqueous zinc‐ion batteries (ZIBs) have attracted extensive interest for the next‐generation batteries, which, however, are facing great challenges due to the poor reversibility of zinc (Zn) anodes and side reactions of water decomposition. Herein, we demonstrated a strategy that the solvation sheath of Zn ions could be facilely regulated by supramolecular coordination chemistry by adding small amounts of cyclodextrins (CDs) and, hence, inhibited the side reactions and side products, widened the electrochemical window, facilitated the homogenous deposition of Zn ions, refined the Zn grains, and enhanced the stability of Zn anodes. Importantly, we demonstrated that compared with α‐ and β‐CD, the γ‐CD showed the best regulation effect of the solvation sheath of Zn ions either at the same molar ratio or at the same mass concentration, which could be ascribed to their difference in supramolecular coordination chemistry and the strongest interaction of γ‐CD with Zn ions. As a result, with γ‐CD, the Zn//Zn symmetric cells showed ultrahigh stability with a cycling lifespan of over 2400 h at a current density of 1 mA/cm2. These results highlight the regulation of solvation sheath by supramolecular coordination chemistry for highly stable Zn anodes and pave a new way to realize high‐performance ZIBs.

2][3][4][5][6][7][8][9] However, the Zn anode faces great challenges in aqueous electrolytes due to the poor reversibility stemming from the formation of Zn dendrites 10 and side reactions of water decomposition. 113][14] The formation of such clusters will aggravate the decomposition of water, and the water molecules in the solvation sheath have been experimentally and theoretically proved to have a lower energy barrier for hydrogen evolution compared with pure water. 15,16][20][21] A lot of methods have been developed to solve such problems, such as follows: (1) the introduction of a protective layer or artificial solid-electrolyte interface for preventing the direct contact of Zn anodes from electrolytes 22-24 ; (2) the fabrication of threedimensional conductive frameworks to accommodate the deposited Zn 25 ; (3) the optimization of the Zn foils by exposing the (002) plane and thus enabling the lateral growth of Zn 26 ; (4) highly concentrated electrolytes, such as "water in salt" or hydrated deep eutectic electrolytes that could widen the electrochemical window and effectively regulate the deposition behavior of metal ions [27][28][29][30] ; (5) the introduction of additives to shielding the Zn surface 31,32 ; and (6) using adsorption of additives on the surface or an ionic selectively permeable membrane to induce homogenous diffusion of Zn ions, 33,34 and so on.
Here, we reported a strategy that the solvation sheath of Zn ions could be facilely regulated by supramolecular coordination chemistry through the addition of small amounts (1 mmol/L if without a special note) of CDs (Figure 1A and Supporting Information: Figure S1) additives into a dilute aqueous 1 mol/L ZnSO 4 (ZS for 1 mol/L ZnSO 4 ) electrolytes.][37] Moreover, CDs possess abundant -OCCObidentate motifs, [38][39][40][41][42] which are favorable to bind divalent Zn ions (Figure 1B).The coordination of CDs with Zn ions was expected to modulate the Zn ions deposition behavior and, hence, inhibit the side reactions, widen the electrochemical window, refine the Zn grains, and enhance the stability of Zn anodes.Simultaneously, the rich hydroxyl groups could also form hydrogen bonds with water molecules and the anions, reduce the concentration gradient, inhibit the formation of the ZHS, and facilitate the homogenous deposition of Zn ions.Importantly, we demonstrated that compared with αand β-CD, the γ-CD showed the best regulation effect of the solvation sheath of Zn ions either at the same molar ratio or at the same mass concentration, which could be ascribed to their difference in supramolecular coordination chemistry and the highest binding energy of γ-CD with Zn ions.As a result, with γ-CD, the Zn//Zn symmetric cells showed ultrahigh stability with a cycling lifespan of over 2400 h at a current density of 1 mA/cm2 .Meanwhile, Zn//V 2 O 5 full cells in ZS electrolyte with γ-CD can maintain 62.2% of capacity after 1500 cycles at 5 A/g, whereas the electrolyte without CDs gave capacity retention of only 36.3%.These results highlight the regulation of solvation sheath by supramolecular coordination chemistry for highly stable Zn anodes and pave a new way to realize high-performance ZIBs.

| RESULTS AND DISCUSSION
In our previous study, we have proven that adjacent functional groups are favorable to the chelation with metal ions. 439][50] Compared with the signals of CDs themselves (Figure 1H and Supporting Information: Figure S3), strong signals at m/z = 518.12,599.15, and 680.17 could be observed in a positive mode, which should be identified to  2+ , respectively.The solvation of Zn 2+ and the modulation of the solvation sheath of Zn 2+ by CDs could also be confirmed by nuclear magnetic resonance.As shown in Figure 1G, the addition of ZS in water shifted the peak of D 2 O from 4.69 ppm to a downfield of 4.71 ppm, which was caused by the strong electron-withdrawing property of Zn 2+ , leading to reduced charge density around the 1 H and confirming the solvation of Zn 2+ in water.However, when different concentrations of γ-CD were added to an aqueous ZS solution, the peak of 1 H showed a slight upfield shift (4.708 and 4.702 ppm for the addition of 1 and 100 mmol/L γ-CD, respectively).The addition of γ-CD led to the coordination of γ-CD with Zn ions and reduced the interaction between Zn ions and water, which undoubtedly reconstructed the solvation sheath of Zn ions.On the other hand, it is noteworthy that without the Zn salt, the low concentrations of γ-CD negligibly perturb the signal of 1 H in D 2 O, indicating that the addition of CDs preferentially coordinated with Zn ions due to the strong electrostatic force. 12,15The strong coordination of CDs with hydrated Zn ions could be further confirmed by the density functional theory (DFT) calculations (Figure 1C, Supporting Information: Figure S4 and Tables S1 and S2).During preparing this paper, two studies have claimed that αor β-CD could be adsorbed on the surface of Zn foil and protect the Zn anodes, via boosting the Zn 2+ deposition kinetics 51 or facilitating the Zn deposition along (002) plane 33 in .relatively high concentrated electrolytes, respectively. 33,51owever, our results indicated that due to the strong electrostatic force, the coordination of CDs with metal ions was ubiquitous in either acidic, basic, or neutral electrolytes (either concentrated or diluted) and such coordination chemistry should be decisive compared with adsorption on the metal surface (metal atoms). 52uch coordination chemistry could also be verified by the strong and ubiquitous electrostatic forces in the electrolytes, for example, the coordination of CDs with other ions and molecules, besides Zn ions.The signals of CDbinded hydrated protons can be detected by HRMS in positive mode (Figure 1H and Supporting Information: Figure S3), indicating the presence of hydrated protons in the weakly acidic ZnSO 4 solution.The addition of CDs and the interactions of CDs with hydrated protons were expected to inhibit the hydrogen evolution to some extent. 27Likewise, the interactions between CDs and anions could also be observed by HRMS in negative mode, due to the presence of rich -OH groups that may form hydrogen bonds with anions (i.e., SO 4 2− ).S5F, 980.4 cm −1 for ZS, whereas 984.3 cm −1 for ZS with 100 mmol/L γ-CD) 12,17 after the addition of γ-CD and the -CH stretching in γ-CD also showed blue shift after the addition of ZS (Supporting Information: Figure S5E), 17 which confirmed the presence of hydrogen bonds between CDs and SO 4 2− .DFT calculations also confirmed the strong coordination of CDs with SO 4 2− (Supporting Information: Figure S6 and Table S3).The interaction of CDs with the anions was expected to inhibit the possible formation of the ZHS and facilitate the homogenous deposition of Zn ions. 55,56ore importantly, the DFT calculations indicated that γ-CD had the strongest binding affinity (−170.07kcal/mol) with Zn ions compared with α-CD (−137.78kcal/mol) and β-CD (−139.33  , showing the smallest distance between γ-CD and Zn ions or SO 4  2− (Supporting Information: Figure S7).
From this point of view, γ-CD should be the best choice compared with αor β-CD to stabilize Zn anodes.
To evaluate the effect of the coordination chemistry and the regulation of the solvation of Zn sheath on the electrochemical performance of the Zn anode, Zn//Ti asymmetric cells (Supporting Information: Figure S8A) were evaluated in ZnSO 4 -based aqueous electrolytes with/without CDs.The linear sweep voltammetry (LSV) at 1 mV/s showed that the current density for the hydrogen evolution reaction decreased significantly after the addition of CDs (−41.56,−29.34, −15.24, and −7.94 mA/cm 2 correspond to ZS and ZS with α-, β-, and γ-CD at −0.2 V, respectively), indicating that the hydrogen evolution was suppressed (Figure 2A). 57uch phenomena could also be observed in the threeelectrode cells (Supporting Information: Figures S9  and S10). 33What is more, the LSV curves showed the most negative onset potential and the smallest current density after the addition of γ-CD, which means that the coordination chemistry of γ-CD could more efficiently modulate the solvation sheath of Zn ions than those of αand β-CD. 26,58In addition, as shown in Figure 2B, the oxygen evolution reaction was also suppressed after the addition of CDs (21.55, 12.6, 10.26, and 8.1 mA/cm 2 correspond to ZS and ZS with α-, β-, and γ-CD at 2.8 V, respectively), which could be ascribed to the formation of hydrogen bonds between the -OH groups of CDs and water molecules as mentioned above. 12,37,51Similarly, γ-CD showed the most significant suppression of the oxygen evolution compared with αand β-CD (Figure 2B).It is obvious that γ-CD has more functional groups than the other two CDs at the same molar ratio.Therefore, to confirm the stronger coordination of γ-CD with the ions than those of the other two CDs, the addition of CDs was controlled at the same mass concentration, which showed the same trend (Supporting Information: Figure S11).The suppression of both hydrogen and oxygen evolution could widen the electrochemical window, in favor of the high capacity and long lifespan of batteries. 31,51Meanwhile, compared with the pristine electrolyte, the addition of CDs significantly reduced the corrosion current density (Figure 2C), which is positively related to the corrosion rate and could be analyzed with the Tafel curves using a three-electrode system. 59Again, the introduction of γ-CD was more conducive to reducing the corrosion current density. 60he absence of CDs resulted in the generation of H 2 bubbles that hindered the deposition of Zn ions after the Tafel tests (Supporting Information: Figure S12).
The coordination of CDs with Zn ions slowed down the deposition rate of Zn ions and probably could enable uniform deposition.The deposition of Zn on Cu foil (Supporting Information: Figure S8B) showed that the nucleation overpotential (η n ) increased from 79.5 mV in ZS to 81.3, 84.4, and 118.2 mV in ZS with α-, β-, and γ-CD, respectively (Figure 2D).And a similar trend was observed on Ti foil (Supporting Information: Figures S8A  and S13).It is well-known that the electrodeposited nuclei size is proportional to the nucleation overpotential, according to the following equation 61,62 : where r is the critical radius of Zn ions; γ is the surface energy between the Zn metal anode and electrolyte interface; V m represents the molar volume of Zn metal; and F corresponds to Faraday's constant.Therefore, the higher nucleation overpotentials indicated the finer grains, which will benefit the homogeneous Zn deposition. 16,33Among the three CDs, the addition of γ-CD again resulted in the largest nucleation overpotential, which was in agreement well with the coordination chemistry as mentioned above.Besides, the different substrates showed a similar trend, indicating that the coordination chemistry rather than the substrate took the more important role.The coordination of CDs with Zn ions retarded the diffusion and the deposition of Zn, which could be verified by the reduced current in the cyclic voltammetry (CV) curves after the addition of CDs (Figure 2E and Supporting Information: Figure S14), the low ionic conductivity (Supporting Information: Table S4), and the increased charge transfer impedance (Figure 2F).All of these results confirmed that the coordination chemistry of CDs could regulate the solvation sheath of Zn ions, widen the electrochemical window, inhibit the side reactions, reduce the deposition rate of Zn ions, refine the Zn grains, and hence should be able to enhance the stability of Zn anodes.The regulation of the solvation sheath of Zn ions by CDs could be confirmed by the morphology of the deposited Zn surface.As shown in Figure 3A-D, the scanning electron microscopic (SEM) images showed that the Zn surfaces after chronoamperometry tests (Figure 3E and Supporting Information: Figure S15) became smooth and flat with the addition of CDs.Specifically, the addition of γ-CD was more conducive to refining the Zn grains.The photo images confirmed this phenomenon with the shining and brighter surface after the addition of CDs (Supporting Information: Figure S16).The addition of the CDs facilitated the homogeneous surface, reduced the tip effect of the grains, and hence the current density of both nucleation and growth would be reduced (Figure 3E and Supporting Information: Figure S15), 25,63 which was coincident well with the previous nucleation overpotential and CV measurements. 64he homogenous deposition of Zn with the addition of CDs should also be ascribed to good contact of electrolyte with Zn anode (Supporting Information: Figure S17), which reduced the surface energy and facilitated the uniform Zn 2+ deposition. 65As mentioned above, the addition of CDs inhibited the side reactions of proton reduction.Consequently, the byproduct ZHS was rarely observed.In contrast, the electrolyte without CDs showed clear peaks of ZHS in the X-ray diffraction patterns (Figure 3F).On the other hand, X-ray photoelectron spectroscopy demonstrated that CDs were not reduced when Zn ions were deposited, as the introduction of γ-CD did not produce any new interfacial components (Supporting Information: Figures S18 and S19).The regulation of solvation sheath of Zn ions by CDs on the Zn deposition could be visually observed using in situ optical microscope (Supporting Information: Figure S20 and Videos S1-S4).The deposition of Zn in different electrolytes was conducted at −1 V for 10 min.Without the addition of CDs, the chaotic diffusion and rapid reduction of Zn ions resulted in rapid growth of dendrites (Figure 3G and Supporting Information: Video S1).Although the introduction of αand β-CD refined the grains, leading to denser and more flat Zn deposition (Figure 3H,I and Supporting Information: Videos S2 and S3), the addition of γ-CD led to very dense Zn deposition with a smooth surface (Figure 3J and Supporting Information: Video S4).A particular note is that the morphology of Zn showed improvements in the sequence of α-< β-< γ-CD, which was coincident well with the aforementioned analyses.This visual observation of the deposition behaviors of Zn ions well validated the previous discussion on the coordination effect of CDs on Zn ions.From this point of view, this strategy could be universal to the deposition of other metal ions.Cu 2+ (0.34 V vs. standard hydrogen electrode) was selected as a proof-of-concept, which confirmed the similar regulation on the metal deposition, showing a flatter surface and finer grain size with the presence of γ-CD (Supporting Information: Figure S21). 67The universality also indicated that the regulation of CDs on the metal deposition should be originated from the coordination mechanism.
The long-term Zn plating/stripping characters were further studied by using Zn//Cu asymmetric devices in different electrolytes and tested by plating (1 mA/cm 2 and 1.1 mAh/cm 2 ) and stripping Zn (0.5 V).In sharp contrast, the γ-CD-contained electrolyte achieved the highest average coulombic efficiency (CE) of 99.1% (98.67% for ZS, 98.89%, and 98.91% for ZS with αand β-CD, respectively) and the longest cycling lifespan of over 200 cycles than those without CDs (91 cycles) or with the addition of αor β-CD (Figure 4C).The results were well coincident with the regulation of solvation sheath of Zn ions by CDs as mentioned above, which could be well demonstrated by the voltage hysteresis of the charge/discharge curve (42.8, 47.8, 55.5, 62.9 mV for ZS, and ZS with α-, β-, γ-CD, respectively, for the 10th cycle as an example).However, the voltage hysteresis increased sharply to 69 mV at the 50th cycle for the Zn// Cu cell without the presence of CDs (the enhancement of 26.2 mV), which was much higher than the electrolytes with CDs.Among them, γ-CD contained electrolytes showed the smallest increase (by 0.2 mV) (Figure 4A,B).Moreover, after 50 cycles, strong diffraction at ~8°could be observed in the cells without CDs (Figure 4D), indicating the appearance of irreversible ZHS.However, only a weak peak was observed in the γ-CD contained electrolyte, indicating that the byproducts were effectively inhibited by CDs, particularly for γ-CD.
The cycling and rate performances were further tested in Zn//Zn symmetric cells.As shown in Figure 4F, under the condition of 1 mA/cm 2 and 1 mAh/cm 2 , without CDs, the Zn//Zn symmetric battery can only cycle for about 760 h.In strong contrast, all CD-contained electrolytes can be stably cycled for more than 1500 h.The addition of γ-CD even promoted stable Zn ions plating/stripping over 2400 h.It is worth mentioning that γ-CD-induced voltage hysteresis in the symmetric battery was still the largest after 300 and 1250 h of cycling compared with αand β-CD.Normally, the presence of overpotential tends to consume more energy.However, the utilization of γ-CD induced a proper overpotential and more importantly it regulated the deposition behaviors of Zn ions, refined the grains and suppressed the irreversible side reactions, leading to improved reversibility of Zn ion plating/stripping. 68hen decreasing (0.5 mA/cm 2 /0.5 mAh/cm 2 ) or increasing (2.5 mA/cm 2 /2.5 mAh/cm 2 , 5 mA/cm 2 /5 mAh/cm 2 ) the current density and areal capacity, the cycling performance of symmetrical cells maintained similar trends, and γ-CD ensured the longest and most stable cycle life (Supporting Information: Figure S22).The CDcontained electrolytes exhibited less voltage fluctuations in the rate performance tests (Figure 4E).The enhancement was superior to most of the previous reports on Zn anode in Supporting Information: Table S5.In addition, after the plating/stripping, clear black residues could be observed on the separators for the electrolytes without CDs, indicating the formation of Zn dendrites (Supporting Information: Figure S23).The SEM images showed rough morphology of Zn surface without CDs and the energydispersive X-ray spectroscopy indicated the presence of large amounts of ZHS with clear S signals (Supporting Information: Figure S24).On the other hand, with the addition of γ-CD, the deposited Zn showed regular shapes of typical Zn crystals and the content of S elements was much lower (Supporting Information: Figure S25).What is more, the distribution of S was much more homogenous, indicating the presence of S was probably in the form of residual electrolytes.
The regulation of Zn ions and the corresponding stability of Zn anodes by γ-CD were evaluated in Zn// V 2 O 5 full-cells (Figure 5A and Supporting Information: Figure S26). 69No additional redox peaks appeared after the addition of γ-CD (Supporting Information: Figure S27), indicating that the addition of γ-CD did not generate other active materials.However, as shown in Figure 5B, the addition of γ-CD resulted in the larger voltage difference (polarization) of the redox pairs of V 2 O 5 (R 1 R 3 > P 1 P 3 , R 2 R 4 > P 2 P 4 ).These results were ZHANG ET AL.
| 7 of 12 consistent well with the galvanostatic charge-discharge (GCD) curves (Figure 5C).In addition, the electrical impedance spectroscopy before and after the CV tests showed that the addition of γ-CD increased the initial charge transfer impedance due to the interaction of γ-CD with anions and cations but stabilized the cells, leading to smaller charge transfer impedance after cycling than that without γ-CD due to the inhibition of the generation of insulating ZHS (Supporting Information: Figure S28). 4,10,70As a result, the electrolytes with γ-CD showed higher rate performance benefiting from the suppression of surface passivation of byproducts (Figure 5D and Supporting Information: Figure S29) and much better stability than those without γ-CD (Figure 5E and Supporting Information: Figure S30). 33,71The slight capacity increase in the initial cycles at 5 A/g should be attributed to the activation of the materials. 9,70,72Impressively, the Zn//V 2 O 5 full cells in 1 mol/L ZnSO 4 electrolyte with γ-CD can maintain 62.2% of capacity after 1500 cycles at 5 A/g, whereas the electrolyte without CDs gave capacity retention of only 36.3%.

| CONCLUSION
In summary, we reported that the supramolecular coordination chemistry of CDs with Zn ions, SO 4 2− anions, and water molecules could regulate the solvation sheath of Zn ions and thereby inhibit the decomposition of water and formation of by-products, widen the electrochemical window, refine the Zn grains, facilitate the homogenous and flat deposition of Zn, and stabilize the Zn anodes.Specifically, due to the strongest coordination of γ-CD with the ions, the addition of γ-CD induced the largest nucleation overpotential and the flattest deposition interface.Consequently, a high average CE of 99.1% and excellent Zn plating/stripping reversibility with an ultralong lifespan of 2400 h were achieved.Furthermore, the cycling stability and rate performances of the Zn//V 2 O 5 full cells using γ-CD are much higher than those without γ-CD.We also demonstrated that the coordination chemistry was universal to the deposition of other metal ions.These results highlight the regulation of solvation sheath by supramolecular coordination chemistry for highly stable Zn anodes and pave a new way to realize highperformance ZIBs.
of the molecular structure of γ-cyclodextrin (γ-CD) and (B) possible coordination sites of γ-CD with metal ions.(C) Binding energy of hydrated Zn ions with different CDs.(D-F) High-resolution mass spectrometry (HRMS) (positive mode) of 0.3 mol/L ZnSO 4 electrolyte with 0.3 mmol/L α-, β-, and γ-CD, respectively, showing the strong coordination of CDs with Zn ions.(G) Proton nuclear magnetic resonance ( 1 H NMR) spectra of D 2 O without and with ZnSO 4 and γ-CD.(H) HRMS (positive mode) of 0.3 mol/L ZnSO 4 electrolyte with 0.3 mmol/L γ-CD, showing the H-bond formation between H in γ-CD with O in H 3 O + .(I) HRMS (negative mode) of 0.3 mol/L ZnSO 4 electrolyte with 0.3 mmol/L γ-CD, showing the H-bond formation between H in γ-CD with O in SO 4 2−

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I G U R E 2 (A, B) The electrochemical stability of electrolytes tested by cathodic and anodic linear sweep voltammetry of Zn//Ti asymmetric cells in different electrolytes, respectively, where the inset of (A) is a magnified view.(C) Corrosion current density of Zn electrode using a three-electrode system (Zn foil as work electrode, Pt as counter electrode, and Ag/AgCl as reference electrode).(D) Comparison of nucleation overpotential by Zn//Cu asymmetric cells.(E, F) Cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS) tested by Zn//Ti asymmetric cells in different electrolytes, respectively.

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I G U R E 3 (A-D) Scanning electron microscopic (SEM) images of Zn foil in ZnSO 4 electrolytes with/without cyclodextrin (α-, β-, and γ-CD) after chronoamperometry (CA) tests for 30 min at −150 mV overpotential versus open circuit voltage (OCV), respectively.(E) CA curves tested by a home-made two electrode configure at −150 mV overpotential for 30 min.(F) X-ray diffraction (XRD) of Zn electrodes after CA tests.(G-J) In situ optical photographs of Zn deposition on Cu foil in ZnSO 4 electrolytes with/without α-, β-, and γ-CD by using CA tests for 10 min at −1 V overpotential versus OCV, respectively.

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I G U R E 4 (A, B) The voltage profiles of the 10th and 50th cycle from CE tests (C) in Zn//Cu asymmetric system in different electrolytes, respectively.(D) X-ray diffraction (XRD) patterns of the Zn-deposited Cu substrates after cycling 50 cycles from CE tests.(E) Rate performances of Zn//Zn cells at different current densities varying from 0.5 to 5 mA/cm 2 .(F) Cycling performance of Zn//Zn symmetric cells using different electrolytes at 1 mA/cm 2 and 1 mAh/cm 2 .

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I G U R E 5 (A) Illustration of Zn//V 2 O 5 full-cells.Comparison of (B) cyclic voltammetry (CV) curves at 0.2 mV/s, (C) galvanostatic charge-discharge curves at 0.2 A/g, (D) rate performance, and (E) cycling stability at 5 A/g of Zn//V 2 O 5 full cells with/without γ-CD, respectively.