The hydrophobic internal cavity and hydrophilic external surface of cyclodextrins (CDs) render promising electrochemical applications. Here, we report a comparative and mechanistic study on the use of CD molecules (α-, β-, and γ-CD) as electrolyte additives for rechargeable Zn batteries. The addition of α-CD in aqueous ZnSO 4 solution reduces nucleation overpotential and activation energy of Zn plating and suppresses H 2 generation. Computational, spectroscopic, and electrochemical studies reveal that α-CD preferentially adsorbs in parallel on the Zn surface via secondary hydroxyl groups, suppressing water-induced side reactions of hydrogen evolution and hydroxide sulfate formation. Additionally, the hydrophilic exterior surface of α-CD with intense electron density simultaneously facilitates Zn 2+ deposition and alleviates Zn dendrite formation. A formulated 3 M ZnSO 4 + 10 mM α-CD electrolyte enables homogenous Zn plating/stripping (average Coulombic efficiency ∼ 99.90%) at 1 mA cm −2 in Zn|Cu cells and a considerable capacity retention of 84.20% after 800 cycles in Zn|V 2 O 5 full batteries. This study provides insight into the use of supramolecular macrocycles to modulate and enhance the interface stability and kinetics of metallic anodes for aqueous battery chemistry.

Rechargeable aqueous Zn batteries are potential for large‐scale electrochemical energy storage due to their low cost and high security. However, Zn metal anode suffers from the dendritic growth and interfacial hydrogen evolution reaction (HER), resulting in the deterioration of electrode/battery performance. Here we propose that both dendrites and HER are related to the water participated Zn2+ solvation structure‐Zn(H2O)62+ and thus can be resolved by transforming Zn(H2O)62+ to an anion‐type water‐free solvation structure‐ZnCl42−, which is achieved in traditional ZnSO4 aqueous electrolyte after adding chloride salt with a bulky cation (1‐ethyl‐3‐methylimidazolium chloride). The elimination of cation‐water interaction suppresses HER, while the electrostatic repulsion between Zn tips and the anion solvation structure inhibits dendrite formation. As a result, the electrolyte shows uniform Zn deposition with an average Zn plating/stripping Coulombic efficiency of ≈99.9 %, enabling a capacity retention of 78.8 % after 300 cycles in anode‐free Zn batteries with pre‐zincificated polyaniline as the cathode. This work provides a novel electrolyte design strategy to prevent HER and realize long‐lifespan metal anode.

Aqueous zinc metal batteries (ZMBs) are considered promising candidates for large-scale energy storage. However, there are still some drawbacks associated with the cathode, zinc anode, and electrolyte that limit their practical application. In this Focus Review, we focus on unveiling the chemical nature of aqueous ZMBs. First, cathode materials and electrochemical reactions are summarized and discussed. Then, fundamental understanding of zinc plating/stripping chemistry is presented, followed by the principle of zinc-dendrite growth and hydrogen evolution as well as the corresponding mitigation strategies. After that, electrolyte engineering, mainly considering salts, concentration, additives, and hydrogel electrolyte, is discussed. Finally, the remaining challenges of aqueous ZMBs toward practical application are noted, mainly including the undefined mechanism and dissolution of the cathode, dendrite growth and hydrogen evolution of the zinc anode, equilibrium between performance and cost of the electrolyte, and testing protocols of full cells. This Focus Review provides new insights for further research and development of aqueous ZMBs.

Metallic Na is a promising anode for rechargeable batteries, however, it is plagued by an unstable solid electrolyte interphase (SEI) and Na dendrites. Herein, a robust anion‐derived SEI is constructed on Na anode in a high‐concentration 1,2‐dimethoxyethane (DME) based electrolyte with a cosolvent hydrofluoroether, which effectively restrains Na dendrite growth. The hydrofluoroether can tune the solvation configuration of the electrolyte from three‐dimensional network aggregates to solvent–cation–anion clusters, enabling more anions to enter and reinforce the inner solvation sheath and their stepwise decomposition. The gradient inorganic‐rich SEI leads to a reduced energy barrier of Na+ migration and enhanced interfacial kinetics. These render the Na||Na3V2(PO4)3 battery with an excellent rate capability of 79.9 mAh g−1 at 24 C and a high capacity retention of 94.2 % after 6000 cycles at 2 C. This highlights the modulation of the electrode–electrolyte interphase chemistry for advanced batteries.

Direct monitoring of dendrite growth, hydrogen evolution, and surface passivation can enrich the chemical and morphological understanding of the unstable Zn/electrolyte interface and provide guidelines for rational design of Zn anodes; however, the on-line observation with high precision is hitherto lacking. Herein, we present a real-time comprehensive characterization system, including in situ atomic force microscopy, optical microscopy, and electrochemical quartz crystal microbalance (referred to as the "3M" system), to provide multiscale views on the semisphere nuclei and growth of bump-like dendrites and the potential-dependent chemical and morphological structures of passivated products in a mild acidic electrolyte. It is revealed that the poor interfacial properties can be attributed to the sparse nucleation sites and direct contact of Zn with the electrolyte. The 3M system further visualizes and confirms that the additive polyethylene glycol acts as a Zn 2+ distribution promoter and physical barrier and merits stable electrochemical performance.