Regulating the Electrical Double Layer with Supramolecular Cyclodextrin Anions for Dendrite-Free Zinc Electrodeposition

Exploring cyclodextrin-driven advancements in aqueous Zn-ion battery: A review

The growing demand for sustainable and efficient energy storage solutions has emphasized the value of aqueous zinc-ion batteries (AZIBs) as a safer and less expensive alternative to lithium-ion batteries. Despite their potential, AZIBs confront considerable obstacles, particularly the production of zinc dendrites and side reactions such hydrogen evolution, which limit their cycling stability and practical applicability. In this review, we explore the prospective function of cyclodextrins (CDs), a type of supramolecular carbohydrate polymer, in addressing such challenges. CDs, with their unique capacity to form inclusion complexes, provide an innovative approach for controlling zinc deposition, limiting dendrite development, and minimizing parasitic interactions. We present a detailed assessment of recent improvements in the use of CDs to change electrolytes and the electrode/electrolyte interface, resulting in improved electrochemical performance of AZIBs. This review focuses on CDs' potential as multifunctional additives for improving zinc anode stability, extending battery life, and facilitating large-scale AZIB adoption.

Anion Receptor-manipulated solvation chemistry and electric double layer enables high Zn-Utilization rate and lean Zn metal batteries

The practical application of aqueous Zn metal batteries (AZMBs) is impeded by inferior reversibility and stability of Zn metal anode (ZMA) originated from side reactions and dendrite growth. Herein, anion receptor l-Proline (LP) is selected to simultaneously manipulate solvation chemistry and electric double layer (EDL) for constructing dendrite-free and stable AZMBs with an ultra-high depth of discharge (DOD of 100 %) and low negative/positive capacity ratio (N/P of 1.1). Experimental and computational results demonstrate that the strong interaction between -SO32- group from OTf- anion and LP promotes the coordination effect of cation and solvent, which improves the stability of electrolyte and induces fine and uniform Zn nucleus formation. Meanwhile, the preferential reduction of OTf- and adsorption of LP establish an anion-derived ZnF2-rich solid electrolyte interface by altering EDL structure, which enhances the mechanical stability and Zn2+ diffusion kinetics of the interface and prevents the contact of H2O molecules. Consequently, ZMA in LP/Zn(OTf)2 electrolyte delivers a satisfactory cycling lifespan under DOD of 100 % and an outstanding Coulombic efficiency of 99.93 % for 10,000 cycles at 10 mA cm-2. Moreover, Zn||Od-NH4V4O10 full cells with LP/Zn(OTf)2 electrolyte demonstrate excellent cycle stability at high cathode loading (20.412 mg), low N/P (1.1), and high temperature (50 °C).

Sieving-type Electric Double Layer with Hydrogen Bond Interlocking to Stable Zinc Metal Anode

The stability of aqueous zinc metal batteries is significantly affected by side reactions and dendrite growth on the anode interface, which primarily originate from water and anions. Herein, we introduce a multi H-bond site additive, 2, 2'-Sulfonyldiethanol (SDE), into an aqueous electrolyte to construct a sieving-type electric double layer (EDL) by hydrogen bond interlock in order to address these issues. On the one hand, SDE replaces H2O and SO4 2- anions that are adsorbed on the zinc anode surface, expelling H2O/SO4 2- from the EDL and thereby reducing the content of H2O/SO4 2- at the interface. On the other hand, when Zn2+ are de-solvated at the interface during the plating, the strong hydrogen bond interaction between SDE and H2O/SO4 2- can trap H2O/SO4 2- from the EDL, further decreasing their content at the interface. This effectively sieves them out of the zinc anode interface and inhibits the side reactions. Moreover, the unique characteristics of trapped SO4 2- anions can restrict their diffusion, thereby enhancing the transference number of Zn2+ and promoting dendrite-free deposition and growth of Zn. Consequently, utilizing an SDE/ZnSO4 electrolyte enables excellent cycling stability in Zn//Zn symmetrical cells and Zn//MnO2 full cells with lifespans exceeding 3500 h and 2500 cycles respectively.

Multifunctional Amphoteric Additive Alanine Enables High-Performance Wide-pH Zn Metal Anodes

Interfacial pH fluctuation is one of the primary reasons for issues related to Zn metal anodes. Herein, polar amphoteric alanine, as a multifunctional electrolyte additive, is designed to regulate the electric double layer (EDL) and solvation structure. Alanine with self-adaptation capability to pH can stabilize electrolyte pH. Due to more negative adsorption energy, alanine preferentially adsorbs on the Zn surface and repels water molecules within the EDL. Alanine-enriched EDL effectively shields the surface tips, homogenizes interfacial electric field distribution, and promotes preferential deposition of horizontal flaky Zn. Alanine-enriched EDL limits the contact between water and the Zn anode. Alanine additive decreases the quantity of water molecules in the Zn2+ solvation sheath and disrupts H-bond networks in the electrolyte. Consequently, a dense and textured Zn deposition is achieved. Corrosion and side reactions are suppressed. Cycling stability of symmetrical cells attains 2700 h at 3 mA cm-2/1 mAh cm-2 and 2050 h at 5 mA cm-2/1 mAh cm-2. Average coulombic efficiency reaches 99.8% over 4500 cycles at 5 mA cm-2/1 mAh cm-2. Even within KOH alkaline electrolytes, alanine additive still improves the cycling lifespan of symmetrical cells to 100 h at 0.5 mA cm-2/0.5 mAh cm-2.

Weakening the Space Charge Layer Effect Through Tethered Anion Electrolyte and Piezoelectric Effect Toward Ultra-Stable Zinc Anode

The space charge layer (SCL) dilemma, caused by mobile anion concentration gradient and the rapid consumption of cations, is the fundamental reason for the generation of zinc dendrites, especially under high-rate discharge conditions. To address the issue, a physical (PbTiO3)/chemical (AMPS-Zn) barrier is designed to construct stable zinc ion flow and disrupt the gradient of anion concentration by coupling the ferroelectric effect with tethered anion electrolyte. The ferroelectric materials PbTiO3 with extreme-high piezoelectric constant can spontaneously generate an internal electric field to accelerate the movement of zinc ions, and the polyanionic polymer AMPS-Zn can repel mobile anions and disrupt the anions concentration gradient by tethering anions. Through numerical simulations and analyses, it is discovered that a high Zn2+ transference number can effectively weaken the SCL, thus suppressing the occurrence of zinc dendrites and parasitic side reactions. Consequently, an asymmetric cell using the PbTiO3@Zn demonstrates a reversible plating/stripping performance for 2900 h, and an asymmetric cell reaches a state-of-the-art runtime of 3450 h with a high average Coulombic efficiency of 99.98%. Furthermore, the PbTiO3@Zn/I2 battery demonstrated an impressive capacity retention rate of 84.0% over 65000 cycles by employing a slender Zn anode.

Synchronous Modulation of H-bond Interaction and Steric Hindrance via Bio-molecular Additive Screening in Zn Batteries

In addressing challenges such as side reaction and dendrite formation, electrolyte modification with bio-molecule sugar species has emerged as a promising avenue for Zn anode stabilization. Nevertheless, considering the structural variability of sugar, a comprehensive screening strategy is meaningful yet remains elusive. Herein, we report the usage of sugar additives as a representative of bio-molecules to develop a screening descriptor based on the modulation of the hydrogen bond component and electron transfer kinetics. It is found that xylo-oligosaccharide (Xos) with the highest H-bond acceptor ratio enables efficient water binding, affording stable Zn/electrolyte interphase to alleviate hydrogen evolution. Meanwhile, sluggish reduction originated from the steric hindrance of Xos contributes to optimized Zn deposition. With such a selected additive in hand, the Zn||ZnVO full cells demonstrate durable operation. This study is anticipated to provide a rational guidance in sugar additive selection for aqueous Zn batteries.

Pyrrolic-Nitrogen Chemistry in 1-(2-hydroxyethyl)imidazole Electrolyte Additives toward a 50,000-Cycle-Life Aqueous Zinc-Iodine Battery

Rechargeable aqueous zinc iodine (Zn-I2) batteries offer benefits such as low cost and high safety. Nevertheless, their commercial application is hindered by hydrogen evolution reaction (HER) and polyiodide shuttle, which result in a short lifespan. In this study, 1-(2-hydroxyethyl)imidazole (HEI) organic molecules featuring pyrrole-N groups are introduced as dually-functional electrolyte additives to simultaneously stabilize Zn anode and confine polyiodide through ion-dipole interactions. The pyrrole-N groups in HEI can preserve the interfacial pH equilibrium at Zn anode by reversibly capturing H+ ions and dynamically neutralizing OH- ions, thereby suppressing the HER. Notably, the H2 evolution rate at the Zn anode is reduced to a mere 2.20 μmol h-1 cm-2. Furthermore, the pyrrole-N moieties in HEI effectively curtail the polyiodide shuttle at I2 cathode, which show adsorption energies of -0.174 eV for I2, -0.521 eV for I3 -, and -0.768 eV for I-, as indicated by density functional theory calculations. Electrochemical testing demonstrates that the Zn//Zn symmetric cell maintains stable cycling for up to 4,200 hours at 1 mA cm-2. Most strikingly, at a high I2 mass loading of 9.7 mg cm-2, the Zn-I2 battery achieves an extraordinary cycle life of 50,000 cycles.

Interfacial Domino Effect Triggered by β-Alanine Cations Realized Highly Reversible Zinc-Metal Anodes

Realizing durative dense, dendrite-free, and no by-product deposition configuration on Zn anodes is crucial to solving the short circuit and premature failure of batteries, which is simultaneously determined by the Zn interface chemistry, electro-reduction kinetics, mass transfer process, and their interaction. Herein, this work unmasks a domino effect of the β-alanine cations (Ala+) within the hydrogel matrix, which effectively triggers the subsequent electrostatic shielding and beneficial knock-on effects via the specifical adsorption earliest event on the Zn anode surface. The electrostatic shielding effect regulates the crystallographic energetic preference of Zn deposits and retards fast electro-reduction kinetics, thereby steering stacked stockier block morphology and realizing crystallographic optimization. Meanwhile, the mass transfer rate of Zn2+ ions was accelerated via the SO4 2- anion immobilized caused by Ala+ in bulk electrolyte, finally bringing the balance between electroreduction kinetics and mass transfer process, which enables dendrite-free Zn deposition behavior. Concomitantly, the interfacial adsorbed Ala+ cations facilitate the electrochemical reduction of interfacial SO4 2- anions to form the inorganic-organic hybrid solid electrolyte interphase layer. The above domino effects immensely improve the utilization efficiency of Zn anodes and long-term stability, as demonstrated by the 12 times longer life of Zn||Zn cells (3650 h) and ultrahigh Coulombic efficiency (99.4 %).

Hofmeister Effects in Supramolecular Chemistry for Anion-Modulation to Stabilize Zn Anode

Aqueous Zn-ion batteries (AZIBs) are considered as promising candidates for the next-generation large-scale energy storage, which, however, is facing the challenge of instable Zn anodes. The anion is pivotal in the stability of anodes, which are not being paid enough attention to. Herein, the modulation of anions is reported using the Hofmeister series in supramolecular chemistry to boost the stability of Zn anodes. It is found that the right-side anions in the Hofmeister series (e.g., OTf-) can enhance the Zn2+ transference number, increase the Coulombic efficiency, facilitate uniform Zn deposition, reduce the freezing point of electrolytes, and thereby stabilize the Zn anodes. More importantly, the right-side anions can form strong interaction with β-cyclodextrin (β-CD) compared to the left-side anions, and hence the addition of β-CD can further enhance the stability of Zn anodes in OTf--based electrolytes, showing enhancement of cycling lifespan in the Zn//Zn symmetric cells more than 45.5 times with β-CD compared with those without β-CD. On the contrary, the left-side anions show worse rate performance after the addition of β-CD. These results provide an effective and novel approach for choosing anions and matching additives to stabilize the anodes and achieve high-performance AZIBs through the Hofmeister effect.

Electric Double Layer Regulator Design through a Functional Group Assembly Strategy towards Long-Lasting Zinc Metal Batteries

Regulating the electric double layer (EDL) structure of the zinc metal anode by using electrolyte additives is an efficient way to suppress interface side reactions and facilitate uniform zinc deposition. Nevertheless, there are no reports investigating the proactive design of EDL-regulating additives before the start of experiments. Herein, a functional group assembly strategy is proposed to design electrolyte additives for modulating the EDL, thereby realizing a long-lasting zinc metal anode. Specifically, by screening ten common functional groups, N, N-dimethyl-1H-imidazole-1-sulfonamide (IS) is designed by assembling an imidazole group, characterized by its high adsorption capability on the zinc anode, and a sulfone group, which exhibits strong binding with Zn2+ ions. Benefiting from the adsorption functionalization of the imidazole group, the IS molecules occupy the position of H2O in the inner Helmholtz layer of the EDL, forming a molecular protective layer to inhibit H2O-induced side reactions. Meanwhile, the sulfone group in IS, acting as a binding site to Zn2+, promotes the de-solvation of Zn2+ ions, facilitating compact zinc deposition. Consequently, the utilization of IS significantly extending the cycling stability of Zn||Zn and Zn||NaV3O8 ⋅ 1.5H2O full cell. This study offers an innovative approach to the design of EDL regulators for high-performance zinc metal batteries.

Design Strategies toward High-Utilization Zinc Anodes for Practical Zinc-Metal Batteries

Aqueous Zn-metal batteries (AZMBs) hold a promise as the next-generation energy storage devices due to their low cost and high specific energy. However, the actual energy density falls far below the requirements of commercial AZMBs due to the use of excessive Zn as anode and the associated issues including dendritic growth and side reactions. Reducing the N/P ratio (negative capacity/positive capacity) is an effective approach to achieve high energy density. A significant amount of research has been devoted to increasing the cathode loading and specific capacity or tuning the Zn anode utilization to achieve low N/P ratio batteries. Nevertheless, there is currently a lack of comprehensive overview regarding how to enhance the utilization of the Zn anode to balance the cycle life and energy density of AZMBs. In this review, we summarize the challenges faced in achieving high-utilization Zn anodes and elaborate on the modifying strategies for the Zn anode to lower the N/P ratio. The current research status and future prospects for the practical application of high-performance AZMBs are proposed at the end of the review.

Multifunctionalized Supramolecular Cyclodextrin Additives Boosting the Durability of Aqueous Zinc-Ion Batteries

The poor cycling stability of aqueous zinc-ion batteries hinders their application in large-scale energy storage due to uncontrollable dendrite growth and harmful hydrogen evolution reactions. Here, we designed and synthesized an electrolyte additive, N-methylimidazolium-β-cyclodextrin p-toluenesulfonate (NMI-CDOTS). The cations of NMI-CD+ are more easily adsorbed on the abrupt Zn surface to regulate the deposition of Zn2+ and reduce dendrite generation under the combined action of the unique cavity structure with abundant hydroxyl groups and the electrostatic force. Meanwhile, p-toluenesulfonate (OTS-) is able to change the Zn2+ solvation structure and suppress the hydrogen evolution reaction by the strong interaction of Zn2+ and OTS-. Benefiting from the synergistic role of NMI-CD+ and OTS-, the Zn||Zn symmetric cell exhibits superior cycling performance as high as 3800 h under 1 mA cm-2 and 1 mA h cm-2. The Zn||V2O5 full battery also shows a high specific capacity (198.3 mA h g-1) under 2.0 A g-1 even after 1500 cycles, and its Coulomb efficiency is nearly 100% during the charging and discharging procedure. These multifunctional composite strategies open up possibilities for the commercial application of aqueous zinc-ion batteries.