Fluoride-Rich, Organic-Inorganic Gradient Interphase Enabled by Sacrificial Solvation Shells for Reversible Zinc Metal Batteries

Tuning Zn-ion de-solvation chemistry with trace amount of additive towards stable Aqueous Zn anodes

Aqueous Zn-ion batteries (AZIBs) have attracted widespread attention due to their intrinsic safety, cost-effectiveness. However, active H2O in the solvated ions [Zn(H2O)6]2+ continuously migrate to the Zn surface to trigger hydrogen evolution reaction (HER) and accelerate Zn corrosion. Herein, Zn dendrites and the related by-products have been successfully inhibited by using trace amounts of Nitrilotriacetic acid (NTA). Theoretical research indicates that two carboxyl groups of NTA molecule strongly anchored on the Zn surface and exposed another carboxyl group outside. Due to the violent interaction of carboxyl groups of NTA with H2O, the de-solvation energy barrier of solvated Zn2+ ([Zn(H2O)6]2+) on the Zn surface was obviously decreased, inhibit the active water splitting. Meanwhile, the preferential adsorption of NTA on the Zn surface increases the thickness of electric double layer EDL and provides a buffer layer to hinder the dendrite growth. Using 0.04 M NTA as additives in 2.0 M ZnSO4 electrolyte, the cycling lifespan of both Zn||Zn symmetric and Zn||MnO2 full cells is markedly prolonged. This study provides certain perspectives for trace amounts of electrolyte additives to satisfy the demand of long-cycle life AZIBs.

Switching Hydrophobic Interface with Ionic Valves for Reversible Zinc Batteries

Developing hydrophobic interface has proven effective in addressing dendrite growth and side reactions during zinc (Zn) plating in aqueous Zn batteries. However, this solution inadvertently impedes the solvation of Zn2+ with H2O and subsequent ionic transport during Zn stripping, leading to insufficient reversibility. Herein, an adaptive hydrophobic interface that can be switched "on" and "off" by ionic valves to accommodate the varying demands for interfacial H2O during both the Zn plating and stripping processes, is proposed. This concept is validated using octyltrimethyl ammonium bromide (C8TAB) as the ionic valve, which can initiatively establish and remove a hydrophobic interface in response to distinct electric-field directions during Zn plating and stripping, respectively. Consequently, the Zn anode exhibits an extended cycling life of over 2500 h with a high Coulombic efficiency of ≈99.8%. The full cells also show impressive capacity retention of over 85% after 1 000 cycles at 5 A g-1. These findings provide a new insight into interface design for aqueous metal batteries.

A Dual Salt/Dual Solvent Electrolyte Enables Ultrahigh Utilization of Zinc Metal Anode for Aqueous Batteries

Rechargeable aqueous zinc batteries are promising in next-generation sustainable energy storage. However, the low zinc (Zn) metal anode reversibility and utilization in aqueous electrolytes due to Zn corrosion and poor Zn2+ deposition kinetics significantly hinder the development of Zn-ion batteries. Here, a dual salt/dual solvent electrolyte composed of Zn(BF4)2/Zn(Ac)2 in water/TEGDME (tetraethylene glycol dimethyl ether) solvents to achieve reversible Zn anode at an ultrahigh depth of discharge (DOD) is developed. An "inner co-salt and outer co-solvent" synergistic effect in this unique dual salt/dual solvent system is revealed. Experimental results and theoretical calculations provide evidence that the ether co-solvent inhibits water activity by forming hydrogen bonding with the water and coordination effects with the proton in the outer Zn2+ solvation structure. Meanwhile, the anion of zinc acetate co-salt enters the inner Zn2+ solvation structure, thereby accelerating the desolvation kinetics. Strikingly, based on the electrolyte design, the zinc anode shows high reversibility at an ultrahigh utilization of 60% DOD with 99.80% Coulombic efficiency and 9.39 mAh cm-2 high capacity. The results far exceed the performance reported in electrolyte design work recently. The work provides fundamental insights into inner co-salt and outer co-solvent synergistic regulation in multifunctional electrolytes for reversible aqueous metal-ion batteries.

Pyrrole as a multi-functional additive to concurrently stabilize Zn anode and cathode via interphase regulation towards advanced aqueous zinc-ion battery

The advancement of aqueous zinc-ion batteries (AZIBs) is impeded by challenges encompassing cathodic and anodic aspects, such as limited capacity and dendrite formation, constraining their broader utilization. Herein, pyrrole, an economically viable and readily accessible compound, is proposed as a versatile electrolyte additive to address these challenges. Experiments and DFT calculations reveal that pyrrole and its derivatives preferentially adsorb onto zinc foil, facilitating the formation of a pyrrole-based solid electrolyte interphase (SEI), which effectively guides uniform Zn2+ deposition through strong attraction force and suppresses hydrogen evolution reactions and parasitic reactions. On the cathode side, the additive promotes the formation of a durable cathode electrolyte interphase (CEI) enriched with poly-pyrrole (Ppy) analogues. Such layer significantly contributes to extra capacity of both polyaniline (PANI) and MnO2 cathodes by leveraging the electrochemical reactivity of Ppy towards Zn2+ and improves their cyclic stability. Consequently, a dendrite-free Zn anode is realized with an extended cyclic lifespan surpassing 6000 h in Zn//Zn cell, coupled with an average Coulombic efficiency of 99.7 % in Cu//Zn cell. Moreover, the PANI//Zn and MnO2//Zn full cells demonstrate enhanced capacities along with improved cycling stability. This work provides a new additive strategy towards concurrent stabilization of cathode and Zn anode in AZIBs.

Trace Small Molecular/Nano-Colloidal Multiscale Electrolyte Additives Enable Ultra-Long Lifespan of Zinc Metal Anodes

Electrolyte additives are efficient to improve the performance of aqueous zinc-ion batteries (AZIBs), yet the current electrolyte additives are limited to fully water-soluble additives (FWAs) and water-insoluble additives (WIAs). Herein, trace slightly water-soluble additives (SWAs) of zinc acetylacetonate (ZAA) were introduced to aqueous ZnSO4 electrolytes. The SWA system of ZAA is composed of a FWA part and a WIA part in a dynamic manner of dissolution equilibrium. The FWA part exists as soluble small molecules, which efficiently regulate Zn2+ ion solvation structure, while the WIA part exists as insoluble nano-colloids, which in-situ form a thick and robust solid electrolyte interface film on zinc metal anodes (ZMAs). Such small molecular/nano-colloidal multiscale electrolyte additives of ZAA are capable to not only improve ionic conductivity and transference number but also inhibit corrosion, hydrogen evolution, and Zn dendrite on ZMAs. The SWA-based Zn∥Zn half battery delivers a superb cumulative plating capacity of 15 Ah cm-2 under 1 mAh cm-2 and 20 mA cm-2, and the SWA-based NH4V4O10∥Zn pouch cell obtains a capacity retention of 67.8% within 4000 cycles under 4 A g-1. The study provides innovative insights for rational design of electrolyte additives, which may pave the way for the practicality of AZIBs.

A Stable High-Performance Zn-Ion Batteries Enabled by Highly Compatible Polar Co-Solvent

Uncontrollable growth of Zn dendrites, irreversible dissolution of cathode material and solidification of aqueous electrolyte at low temperatures severely restrict the development of aqueous Zn-ion batteries. In this work, 2,2,2-trifluoroethanol (TFEA) with a volume fraction of 50% as a highly compatible polar-solvent is introduced to 1.3 M Zn(CF3SO3)2 aqueous electrolyte, achieving stable high-performance Zn-ion batteries. Massive theoretical calculations and characterization analysis demonstrate that TFEA weakens the tip effect of Zn anode and restrains the growth of Zn dendrites due to electrostatic adsorption and coordinate with H2O to disrupt the hydrogen bonding network in water. Furthermore, TFEA increases the wettability of the cathode and alleviates the dissolution of V2O5, thus improving the capacity of the full battery. Based on those positive effects of TFEA on Zn anode, V2O5 cathode, and aqueous electrolyte, the Zn//Zn symmetric cell delivers a long cycle-life of 782 h at 5 mA cm-2 and 2 mA h cm-2. The full battery still declares an initial capacity of 116.78 mA h g-1, and persists 87.73% capacity in 2000 cycles at -25 °C. This work presents an effective strategy for fully compatible co-solvent to promote the stability of Zn anode, V2O5 cathode and aqueous electrolyte for high-performance Zn-ion batteries.

Superfast Zincophilic Ion Conductor Enables Rapid Interfacial Desolvation Kinetics for Low-Temperature Zinc Metal Batteries

Low-temperature rechargeable aqueous zinc metal batteries (AZMBs) as highly promising candidates for energy storage are largely hindered by huge desolvation energy barriers and depressive Zn2+ migration kinetics. In this work, a superfast zincophilic ion conductor of layered zinc silicate nanosheet (LZS) is constructed on a metallic Zn surface, as an artificial layer and ion diffusion accelerator. The experimental and simulation results reveal the zincophilic ability and layer structure of LZS not only promote the desolvation kinetics of [Zn(H2O)6]2+ but also accelerate the Zn2+ transport kinetics across the anode/electrolyte interface, guiding uniform Zn deposition. Benefiting from these features, the LZS-modified Zn anodes showcase long-time stability (over 3300 h) and high Coulombic efficiency with ≈99.8% at 2 mA cm-2, respectively. Even reducing the environment temperature down to 0 °C, ultralong cycling stability up to 3600 h and a distinguished rate performance are realized. Consequently, the assembled Zn@LZS//V2O5-x full cells deliver superior cyclic stability (344.5 mAh g-1 after 200 cycles at 1 A g-1) and rate capability (285.3 mAh g-1 at 10 A g-1) together with a low self-discharge rate, highlighting the bright future of low-temperature AZMBs.

Electrolyte Solvation Chemistry for Stabilizing the Zn Anode via Functionalized Organic Agents

As a potential candidate for grid-scale energy storage technology, aqueous Zn-ion batteries (ZIBs) have attracted considerable attention due to their intrinsic safety, environmental friendliness, and ease of fabrication. Nevertheless, the road to industry for this technique is hindered by serious issues, including undesired side reactions, random growth of the Zn dendrites, electrode passivation, and anode corrosion, which are associated with the high reactivity of water molecules during the electrochemical reactions. These challenges are strongly dependent on electrolyte solvation chemistry (ESC), which subsequently determines the electrochemical behavior of the metal ions and water molecules on the electrode surface. In this work, a comprehensive understanding of optimized ESC with specified functional groups on the mixing agents to stabilize the Zn anode is provided. First, the challenges facing the ZIBs and their chemical principles are outlined. Specific attention is paid to the working principles of the mixing agents with different functional groups. Then the recent progress is summarized and compared. Finally, perspectives on future research for the aqueous Zn batteries are presented from the point of view.

Multi-Group Polymer Coating on Zn Anode for High Overall Conversion Efficiency Photorechargeable Zinc-Ion Batteries

The solar-driven photorechargeable zinc-ion batteries have emerged as a promising power solution for smart electronic devices and equipment. However, the subpar cyclic stability of the Zn anode remains a significant impediment to their practical application. Herein, poly(diethynylbenzene-1,3,5-triimine-2,4,6-trione) (PDPTT) was designed as a functional polymer coating of Zn. Theoretical calculations demonstrate that the PDPTT coating not only significantly homogenizes the electric field distribution on the Zn surface, but also promotes the ion-accessible surface of Zn. With multiple N and C=O groups exhibiting strong adsorption energies, this polymer coating reduces the nucleation overpotential of Zn, alters the diffusion pathway of Zn2+ at the anode interface, and decreases the corrosion current and hydrogen evolution current. Leveraging these advantages, Zn-PDPTT//Zn-PDPTT exhibits an exceptionally long cycling time (≥4300 h, 1 mA cm-2). Zn-PDPTT//AC zinc-ion hybrid capacitors can withstand 50,000 cycles at 5 A/g. Zn-PDPTT//NVO zinc-ion battery exhibits a faster charge storage rate, higher capacity, and excellent cycling stability. Coupling Zn-PDPTT//NVO with high-performance perovskite solar cells results in a 13.12 % overall conversion efficiency for the photorechargeable zinc-ion battery, showcasing significant value in advancing the efficiency and upgrading conversion of renewable energy utilization.

Regulating Zn2+ Migration-Diffusion Behavior by Spontaneous Cascade Optimization Strategy for Long-Life and Low N/P Ratio Zinc Ion Batteries

Parasitic side reactions and dendrite growth on zinc anodes are formidable issues causing limited lifetime of aqueous zinc ion batteries (ZIBs). Herein, a spontaneous cascade optimization strategy is first proposed to regulate Zn2+ migration-diffusion behavior. Specifically, PAPE@Zn layer with separation-reconstruction properties is constructed in situ on Zn anode. In this layer, well-soluble poly(ethylene oxide) (PEO) can spontaneously separation to bulk electrolyte and weaken the preferential coordination between H2O and Zn2+ to achieve primary optimization. Meanwhile, poor-soluble polymerized-4-acryloylmorpholine (PACMO) is reconstructed on Zn anode as hydrophobic flower-like arrays with abundant zincophilic sites, further guiding the de-solvation and homogeneous diffusion of Zn2+ to achieve the secondary optimization. Cascade optimization effectively regulates Zn2+ migration-diffusion behavior, dendrite growth and side reactions of Zn anode are negligible, and the stability is significantly improved. Consequently, symmetrical cells exhibit stability over 4000 h (1 mA cm-2). PAPE@Zn//NH4 +-V2O5 full cells with a high current density of 15 A g-1 maintains 72.2 % capacity retention for 12000 cycles. Even better, the full cell demonstrates excellent performance of cumulative capacity of 2.33 Ah cm-2 at ultra-low negative/positive (N/P) ratio of 0.6 and a high mass-loading (~17 mg cm-2). The spontaneous cascade optimization strategy provides novel path to achieve high-performance and practical ZIBs.

A Multifunctional Additive Based on the Cation-Anion Synergistic Effect for Highly Stable Zinc Metal Anodes

The Zn dendrite and hydrogen evolution reaction have been a "stubborn illness" for the life span of zinc anodes, which significantly hinders the development of aqueous zinc batteries (AZBs). Herein, considering the ingenious molecular structure, a multifunctional additive based on the synergistic regulation of cations and anions at the interface is designed to promote a dendrite-free and stable Zn anode. Theoretical calculations and characterization results verified that the electrostatic shield effect of the cation, the solvation sheath structure, and the bilayer structural solid electrolyte film (SEI) jointly account for the uniform Zn deposition and side reaction suppression. Ultimately, a remarkably high average Coulombic efficiency (CE) of 99.4% is achieved in the Zn||Cu cell for 300 cycles, and a steady charge/discharge cycling over 3000 and 300 h at 1.0 mA cm-2/1.0 mAh cm-2 and 10 mA cm-2/10 mAh cm-2 is obtained in the Zn||Zn cell. Furthermore, the assembled full battery demonstrates a prolonged cycle life of 2000 cycles.

Molecular Engineering Enables Hydrogel Electrolyte with Ionic Hopping Migration and Self-Healability toward Dendrite-Free Zinc-Metal Anodes

Hydrogel electrolytes (HEs), characterized by intrinsic safety, mechanical stability, and biocompatibility, can promote the development of flexible aqueous zinc-ion batteries (FAZIBs). However, current FAZIB technology is severely restricted by the uncontrollable dendrite growth arising from undesirable reactions between the HEs with sluggish ionic conductivity and Zn metal. To overcome this challenge, this work proposes a molecular engineering strategy, which involves the introduction of oxygen-rich poly(urea-urethane) (OR-PUU) into polyacrylamide (PAM)-based HEs. The OR-PUU/PAM HEs facilitate rapid ion transfer through their ionic hopping migration mechanism, resulting in uniform and orderly Zn2+ deposition. The abundant polar groups on the OR-PUU molecules in OR-PUU/PAM HEs break the inherent H-bond network, tune the solvation structure of hydrated Zn2+, and inhibit the occurrence of side reactions. Moreover, the interaction of hierarchical H-bonds in the OR-PUU/PAM HEs endows them with self-healability, enabling in situ repair of cracks induced by plating/stripping. Consequently, Zn symmetric cells incorporating the novel OR-PUU/PAM HEs exhibit a long cycling life of 2000 h. The resulting Zn-MnO2 battery displays a low capacity decay rate of 0.009% over 2000 cycles at 2000 mA g-1. Overall, this work provides valuable insights to facilitate the realization of dendrite-free Zn-metal anodes through the molecular engineering of HEs.

Cation-Conduction Dominated Hydrogels for Durable Zinc-Iodine Batteries

Zinc-iodine batteries have the potential to offer high energy-density aqueous energy storage, but their lifetime is limited by the rampant dendrite growth and the concurrent parasite side reactions on the Zn anode, as well as the shuttling of polyiodides. Herein, a cation-conduction dominated hydrogel electrolyte is designed to holistically enhance the stability of both zinc anode and iodine cathode. In this hydrogel electrolyte, anions are covalently anchored on hydrogel chains, and the major mobile ions in the electrolyte are restricted to be Zn2+. Specifically, such a cation-conductive electrolyte results in a high zinc ion transference number (0.81) within the hydrogel and guides epitaxial Zn nucleation. Furthermore, the optimized Zn2+ solvation structure and the reconstructed hydrogen bond networks on hydrogel chains contribute to the reduced desolvation barrier and suppressed corrosion side reactions. On the iodine cathode side, the electrostatic repulsion between negative sulfonate groups and polyiodides hinders the loss of the iodine active material. This all-round electrolyte design renders zinc-iodine batteries with high reversibility, low self-discharge, and long lifespan.

Toward a Metal Anode-Free Zinc-Air Battery for Next-Generation Energy Storage

Rechargeable aqueous zinc-air batteries (ZABs) promise high energy density and safety. However, the use of conventional zinc anodes affects the energy output from the battery, so that the theoretical energy density is not achievable under operation conditions. A large portion of the zinc is shielded by anode passivation during the discharge process and remains electrochemically unused, making the operation of rechargeable ZABs inefficient up to date. In a metal anode-free ZAB, there is no unnecessary excess zinc if the zinc reservoir can be precisely adjusted by electrodeposition of zinc from the electrolyte. In this respect, an anode-free battery uses the electrolyte offering a dual-mode functionality not only providing ionic conductivity but also being the source of zinc. In addition, it is shown that a defined porous anode architecture is crucial for high rechargeability in this new type of ZAB. 3D-spatially arranged carbon nanotubes as geometrically defined host structures allow a homogeneous zinc deposition from the electrolyte. Together with carbon nanohorns as an active 2e- catalyst on the cathode side, the rechargeability of this new concept reaches up to 92%.

Dynamic Molecular Interphases Regulated by Trace Dual Electrolyte Additives for Ultralong-Lifespan and Dendrite-Free Zinc Metal Anode

Metallic zinc is a promising anode material for rechargeable aqueous multivalent metal-ion batteries due to its high capacity and low cost. However, the practical use is always beset by severe dendrite growth and parasitic side reactions occurring at anode/electrolyte interface. Here we demonstrate dynamic molecular interphases caused by trace dual electrolyte additives of D-mannose and sodium lignosulfonate for ultralong-lifespan and dendrite-free zinc anode. Triggered by plating and stripping electric fields, the D-mannose and lignosulfonate species are alternately and reversibly (de-)adsorbed on Zn metal, respectively, to accelerate Zn2+ transportation for uniform Zn nucleation and deposition and inhibit side reactions for high Coulombic efficiency. As a result, Zn anode in such dual-additive electrolyte exhibits highly reversible and dendrite-free Zn stripping/plating behaviors for >6400 hours at 1 mA cm-2, which enables long-term cycling stability of Zn||ZnxMnO2 full cell for more than 2000 cycles.

Artificial Hydrophilic Organic and Dendrite-Suppressed Inorganic Hybrid Solid Electrolyte Interface Layer for Highly Stable Zinc Anodes

Aqueous zinc-ion batteries (AZIBs) have gained significant attentions for their inherent safety and cost-effectiveness. However, challenges, such as dendrite growth and anodic corrosion at the Zn anode, hinder their commercial viability. In this paper, an organic-inorganic coating layer (Nafion-TiO2) was introduced to protect the Zn anode and electrolyte interface. Briefly, Nafion effectively shields against the corrosion from water molecules through the hydrophobic wall of -CF3 and guided zinc deposition from the -SO3 functional group, while TiO2 particles with a higher Young's modulus (151 GPa vs 120 GPa from Zn metal) suppress the zinc dendrite formation. As a result, with the protection of Nafion-TiO2, the symmetrical Zn∥Zn battery shows an improved cycle life of 1,750 h at 0.5 mA cm-2, and the full cell based on Zn∥MnO2 shows a long cycle life over 1,500 cycles at 1 A g-1. Our research offers a novel approach for protecting zinc metal anodes, potentially applicable to other metal anodes such as those in lithium and sodium batteries.

Constructing a Topologically Adaptable Solid Electrolyte Interphase for a Highly Reversible Zinc Anode

The performance of aqueous zinc metal batteries is significantly compromised by the stability of the solid electrolyte interphase (SEI), which is intimately linked to the structure of the electrical double layer (EDL) between the zinc anode and electrolyte. Furthermore, understanding the mechanical behavior of SEI is crucial, as it governs its response to stress induced by volume changes, fracture, or deformation. In this study, we introduce l-glutamine (Gln) as an additive to regulate the adsorbed environment of the EDL and in situ produce a hybrid SEI consisting of ZnS and Gln-related species. The results of the nanoindentation test indicate that the hybrid SEI exhibits a low modulus and low hardness, alongside exceptional shape recovery capability, which effectively limits side reactions and enables topological adaptation to volume fluctuations in zinc anodes during zinc ion plating/stripping, thereby enabling Zn//Zn symmetric cells to exhibit an ultralong cycle life of 4000 h in coin cells and a high cumulative capacity of 18,000 mA h in pouch cells. More importantly, the superiority of the formulated strategy is further demonstrated in Zn//NH4V4O10 full cells at different N/P ratios of 5.2, 4.9, 3.5, and 2.4. This provides a promising approach for future interfacial modulation in aqueous battery chemistry.