Towards More Sustainable Aqueous Zinc-Ion Batteries.

Aqueous zinc-ion batteries (AZIBs) are considered as the promising candidates for large-scale energy storage because of their high safety, low cost and environmental benignity. The large-scale applications of AZIBs will inevitably result in a large amount of spent AZIBs, which not only induce the waste of resources, but also pose environmental risks. Therefore, sustainable AZIBs have to be considered to minimize the risk of environmental pollution and maximize the utilization of spent compounds. Herein, this minireview focuses on the sustainability of AZIBs from material design and recycling techniques. The structure and degradation mechanism of AZIBs are discussed to guide the recycling design of the materials. Subsequently, the sustainability of component materials in AZIBs is further analysed to pre-evaluate their recycling behaviors and mentor the selection of more sustainable component materials, including active materials in cathodes, Zn anodes, and aqueous electrolytes, respectively. According to the features of component materials, corresponding green and economic approaches are further proposed to realize the recycling of active materials in cathodes, Zn anodes and electrolytes, respectively. These advanced technologies endow the recycling of component materials with high efficiency and a closed-loop control, ensuring that AZIBs will be the promising candidates of sustainable energy storage devices. This review will offer insight into potential future directions in the design of sustainable AZIBs.

The Construction of Anion-Induced Solvation Structures in Low-concentration Electrolyte for Stable Zinc Anodes.

Aqueous zinc-ion batteries (ZIBs) are promising large-scale energy storage devices because of their low cost and high safety. However, owing to the high activity of H2O molecules in electrolytes, hydrogen evolution reaction and side reactions usually take place on Zn anodes. Herein, additive-free PCA-Zn electrolyte with capacity of suppressing the activity of free and solvated H2O molecules was designed by selecting the cationophilic and solventophilic anions. In such electrolyte, contact ion-pairs and solvent-shared ion-pairs were achieved even at low concentration, where PCA- anions coordinate with Zn2+ and bond with solvated H2O molecules. Simultaneously, PCA- anions also induce the construction of H-bonds between free H2O molecules and them. Therefore, the activity of free and solvated H2O molecules is effectively restrained. Furthermore, since PCA- anions possess a strong affinity with metal Zn, they can also adsorb on Zn anode surface to protect Zn anode from the direct contact of H2O molecules, inhibiting the occurrence of water-triggered side reactions. As a result, plating/stripping behavior of Zn anodes is highly reversible and the coulombic efficiency can reach to 99.43 % in PCA-Zn electrolyte. To illustrate the feasibility of PCA-Zn electrolyte, the Zn||PANI full batteries were assembled based on PCA-Zn electrolyte and exhibited enhanced cycling performance.

A High-Energy Aqueous All-Sulfur Battery.

Rechargeable aqueous batteries are promising energy storage devices because of their high safety and low cost. However, their energy densities are generally unsatisfactory due to the limited capacities of ion-inserted electrode materials, prohibiting their widespread applications. Herein, a high-energy aqueous all-sulfur battery was constructed via matching S/Cu2 S and S/CaSx redox couples. In such batteries, both cathodes and anodes undergo the conversion reaction between sulfur/metal sulfides redox couples, which display high specific capacities and rational electrode potential difference. Furthermore, during the charge/discharge process, the simultaneous redox of Cu2+ ion charge-carriers also takes place and contributes to a more two-electron transfer, which doubles the capacity of cathodes. As a result, the assembled aqueous all-sulfur batteries deliver a high discharge capacity of 447 mAh g-1 based on total mass of sulfur in cathode and anode at 0.1 A g-1 , contributing to an enhanced energy density of 393 Wh kg-1 . This work will widen the scope for the design of high-energy aqueous batteries.

Six-Electron-Redox Iodine Electrodes for High-Energy Aqueous Batteries.

Iodine (I2 ) shows great promising as the active material in aqueous batteries due to its distinctive merits of high abundance in ocean and low cost. However, in conventional aqueous I2 -based batteries, the energy storage mechanism of I- /I2 conversion is only two-electron redox reaction, limiting their energy density. Herein, six-electron redox chemistry of I2 electrodes is achieved via the synergistic effect of redox-ion charge-carriers and halide ions in electrolytes. The redox-active Cu2+ ions in electrolytes induce the conversion between Cu2+ ions and I2 to CuI at low potential. Simultaneously, the Cl- ions in electrolytes activate the I2 /ICl redox couple at high potential. As a result, in our case, I2 -based battery system with six-electron redox is developed. Such energy storage mechanism with six-electron redox leads to high discharge potential and capacity, excellent rate capability, as well as stable cycling behavior of I2 electrodes. Impressively, six-electron-redox I2 cathodes can match various aqueous metal (e.g. Zn, Mn and Fe) anodes to construct metal||I2 hybrid batteries. These hybrid batteries not only deliver enhanced capacities, but also exhibit higher operate voltages, which contributes to superior energy densities. Therefore, this work broadens the horizon for the design of high-energy aqueous I2 -based batteries.

High-Energy Aqueous/Organic Hybrid Batteries Enabled by Cu2+ Redox Charge Carriers.

Lithium||sulfur (Li||S) batteries are considered as one of the promising next-generation batteries due to the high theoretical capacity and low cost of S cathodes, as well as the low redox potential of Li metal anodes (-3.04 V vs. standard hydrogen electrode). However, the S reduction reaction from S to Li2 S leads to limited discharge voltage and capacity, largely hindering the energy density of Li||S batteries. Herein, high-energy Li||S hybrid batteries were designed via an electrolyte decoupling strategy. In cathodes, S electrodes undergo the solid-solid conversion reaction from S to Cu2 S with four-electron transfer in a Cu2+ -based aqueous electrolyte. Such an energy storage mechanism contributes to enhanced electrochemical performance of S electrodes, including high discharge potential and capacity, superior rate performance and stable cycling behavior. As a result, the assembled Li||S hybrid batteries exhibit a high discharge voltage of 3.4 V and satisfactory capacity of 2.3 Ah g-1 , contributing to incredible energy density. This work provides an opportunity for the construction of high-energy Li||S batteries.

Coupling dual metal active sites and low-solvation architecture toward high-performance aqueous ammonium-ion batteries.

Aqueous rechargeable ammonium-ion batteries (AIBs) possess the characteristics of safety, low cost, environmental friendliness, and fast diffusion kinetics. However, their energy density is often limited due to the low specific capacity of cathode materials and narrow electrochemical stability windows of electrolytes. Herein, high-performance aqueous AIBs were designed by coupling Fe-substituted manganese-based Prussian blue analog (FeMnHCF) cathodes and highly concentrated NH4CF3SO3 electrolytes. In FeMnHCF, Mn3+/Mn2+-N redox reaction at high potential was introduced, and two metal active redox species of Mn and Fe were achieved. To match such FeMnHCF cathodes, highly concentrated NH4CF3SO3 electrolyte was further developed, where NH4+ ion displays low-solvation structure because of the increased coordination number of CF3SO3- anions. Furthermore, the water molecules are confined by NH4+ and CF3SO3- ions in their solvation sheath, leading to weak interaction between water molecules and thus effectively extending the voltage window of electrolyte. Consequently, the FeMnHCF electrodes present high reversibility during the charge/discharge process. Moreover, owing to a small amount of free water in concentrated electrolyte, the dissolution of FeMnHCF is also inhibited. As a result, the assembled aqueous AIBs exhibit enhanced energy density, excellent rate capability, and stable cycling behavior. This work provides a creative route to construct high-performance aqueous AIBs.

Proton Chemistry Induced Long-Cycle Air Self-Charging Aqueous Batteries.

Air self-charging aqueous metal-ion batteries usually suffer from capacity loss after self-charging cycles due to the formation of basic salts on cathodes in the near-neutral electrolytes. Here, air self-charging Pb/pyrene-4,5,9,10-tetraone (PTO) batteries based on proton chemistry are developed in acidic electrolyte. The fast kinetics of H+ uptake/removal endows the battery with enhanced electrochemical performance. Owing to the high standard electrode potential of oxygen in acid electrolyte, the discharged cathodes are spontaneously oxidized by oxygen in air along with H+ extraction and thus achieve self-charging without external power supply. Notably, the air self-charging mechanism involved H+ -based redox can effectively avoid the generation of basic salts on self-charging electrodes and thus guarantee long-term self-charging/galvanostatic discharging cycles of Pb/PTO batteries. This work provides a promising strategy for designing long-cycle air self-charging systems.

An Air-Rechargeable Zn/Organic Battery with Proton Storage.

Air-rechargeable zinc batteries are a promising candidate for self-powered battery systems since air is ubiquitous and cost-free. However, they are still in their infancy and their electrochemical performance is unsatisfactory due to the bottlenecks of materials and device design. Therefore, it is of great significance to develop creative air-rechargeable Zn battery systems. Herein, an air-rechargeable Zn battery with H+-based chemistry was developed in a mild ZnSO4 electrolyte for the first time, where benzo[i]benzo[6,7]quinoxalino[2,3-a]benzo[6,7]quinoxalino[2,3-c]phenazine-5,8,13,16,21,24-hexaone (BQPH) was employed as cathode material. In this Zn/BQPH battery, a Zn2+ coordination with adjacent C═O and C═N groups leads to an inhomogeneous charge distribution in the BQPH molecule, which induces the H+ uptake on the remaining four pairs of the C═O and C═N groups in subsequent discharge processes. Interestingly, the large potential difference between the discharged cathode of the Zn/BQPH battery and oxygen triggers the redox reaction between them spontaneously, in which the discharged cathode can be oxidized by oxygen in air. In this process, the cathode potential will gradually rise along with H+ removal, and the discharged Zn/BQPH battery can be air-recharged without an external power supply. As a result, the air-rechargeable Zn/BQPH batteries exhibit enhanced electrochemical performance by fast H+ uptake/removal. This work will broaden the horizons of air-rechargeable zinc batteries and provide a guidance to develop high-performance and sustainable aqueous self-powered systems.

A Lattice-Matching Strategy for Highly Reversible Copper-Metal Anodes in Aqueous Batteries.

Copper metal is an attractive anode material for aqueous rechargeable batteries due to its high theoretical specific capacity (844 mAh g-1 ), good environmental compatibility and high earth abundance. However, the Cu anodes often suffer from poor deposition/stripping reversibility and nonuniform deposition during the charge/discharge process, degrading the lifetime of aqueous Cu-metal batteries. Herein, a lattice-matching strategy was developed to design high-performance Cu-metal anodes. In such a strategy, Ni substrates that exhibit high lattice matching with Cu were selected to support the Cu anodes. The high lattice matching endows Cu anodes with high deposition/stripping reversibility, low nucleation overpotential as well as a uniform and dense electrodeposition on Ni substrates. Based on the Ni substrate-supported Cu anodes, the full cells paired with lead dioxide cathodes show a stable cycling behavior. This work provides a route for the design of high-performance Cu electrodes in aqueous rechargeable batteries.

A Binary Hydrate-Melt Electrolyte with Acetate-Oriented Cross-Linking Solvation Shells for Stable Zinc Anodes.

Aqueous zinc-ion batteries (ZIBs) with low cost and high safety are promising energy-storage devices. However, ZIBs with metal Zn anodes usually suffer from low coulombic efficiency and poor cycling performance due to the occurrence of side reactions on the Zn anodes. Here, a binary hydrate-melt ZnCl2 /Zn(OAc)2 electrolyte is designed to suppress the hydrogen evolution reaction and by-product formation on Zn anodes by adjusting the Zn2+ solvation structure. In the solvation structure of the hydrate-melt ZnCl2 /Zn(OAc)2 electrolyte, the carboxylate group in OAc- will coordinate with the Zn2+ , which will weaken the interaction between Zn2+ and H2 O molecules to achieve higher ionization energy of H2 O molecules. Simultaneously, these carboxylate groups of OAc- can serve as H-bond acceptors to construct H-bonds with H2 O molecules in their neighboring solvation structures, forming a cross-linking H-bond network. Such a cross-linking H-bond network further suppresses the water activity in ZnCl2 /Zn(OAc)2 electrolyte. As a result, in such an electrolyte, the side reactions are effectively restricted on Zn anodes and thus Zn anodes can achieve a high coulombic efficiency of 99.59% even after cycling. To illustrate the feasibility of the ZnCl2 /Zn(OAc)2 electrolyte in aqueous ZIBs, Zn||p-chloranil cells are assembled based on the ZnCl2 /Zn(OAc)2 electrolyte. The resultant Zn||p-chloranil cells exhibit enhanced cycling performance compared with the cases with a conventional ZnSO4 electrolyte.

Proton-Assisted Aqueous Manganese-Ion Battery Chemistry.

Aqueous manganese-ion batteries (MIBs) are promising energy storage systems because of the distinctive merits of Mn metal, in terms of high abundance, low cost, nontoxicity, high theoretical capacity and low redox potential. Conventional MIBs are based on the Mn2+ ion storage mechanism, whereas the capacity in cathode materials is generally limited due to the high charge density and large solvated ionic radius of Mn2+ ions in aqueous electrolytes. Herein, proton intercalation chemistry is introduced in aqueous MIBs, in which the layered Al0.1 V2 O5 ⋅1.5 H2 O (AlVO) cathode exhibits a consequent Mn2+ and H+ ion intercalation/extraction process. Such an energy storage mechanism contributes to enhanced electrochemical performance, including high capacity, fast reaction kinetics and stable cycling behavior. Benefiting from this proton intercalation chemistry, the aqueous Mn||AlVO cells could deliver high specific energy and power simultaneously. This work provides a route for the design of high-performance aqueous MIBs.

A rechargeable aqueous manganese-ion battery based on intercalation chemistry.

Aqueous rechargeable metal batteries are intrinsically safe due to the utilization of low-cost and non-flammable water-based electrolyte solutions. However, the discharge voltages of these electrochemical energy storage systems are often limited, thus, resulting in unsatisfactory energy density. Therefore, it is of paramount importance to investigate alternative aqueous metal battery systems to improve the discharge voltage. Herein, we report reversible manganese-ion intercalation chemistry in an aqueous electrolyte solution, where inorganic and organic compounds act as positive electrode active materials for Mn2+ storage when coupled with a Mn/carbon composite negative electrode. In one case, the layered Mn0.18V2O5·nH2O inorganic cathode demonstrates fast and reversible Mn2+ insertion/extraction due to the large lattice spacing, thus, enabling adequate power performances and stable cycling behavior. In the other case, the tetrachloro-1,4-benzoquinone organic cathode molecules undergo enolization during charge/discharge processes, thus, contributing to achieving a stable cell discharge plateau at about 1.37 V. Interestingly, the low redox potential of the Mn/Mn2+ redox couple vs. standard hydrogen electrode (i.e., -1.19 V) enables the production of aqueous manganese metal cells with operational voltages higher than their zinc metal counterparts.

A Universal Compensation Strategy to Anchor Polar Organic Molecules in Bilayered Hydrated Vanadates for Promoting Aqueous Zinc-Ion Storage.

The electrochemical performance of layered vanadium oxides is often improved by introducing guest species into their interlayer. Guest species with high stability in the interlayer and weak interaction with Zn2+ during charge/discharge process are desired to promoting reversible Zn2+ transfer. Herein, a universal compensation strategy was developed to introduce various polar organic molecules into the interlayer of Alx V2 O5 ·nH2 O by replacing partial crystal water. The high-polar groups in the organic molecules have a strong electrostatic attraction with pre-intercalated Al3+ , which ensures that organic molecules can be anchored in the interlayer of hydrated vanadates. Simultaneously, the low-polar groups endow organic molecules with a weak interaction with Zn2+ during cycling, thus liberalizing reversible Zn2+ transfer. As a result, Alx V2 O5 with polar organic molecules displays enhanced electrochemical performance. Furthermore, based on above cathode material, a pouch cell was assembled by further integrating a dendrite-free N-doped carbon nanofiber@Zn anode, displaying an energy density of 50 Wh kg-1 . This work provides a path for designing stable guest species with a weak interaction with Zn2+ in the interlayer of layered vanadium oxide towards high-performance cathode materials of aqueous Zn batteries.

Non-Metal Ion Co-Insertion Chemistry in Aqueous Zn/MnO2 Batteries.

The co-insertion of dual ions can often offer enhanced electrochemical performance for the aqueous zinc batteries. Although the insertion of non-metallic ions has been achieved in aqueous zinc batteries, the co-insertion chemistry of non-metallic cations is still a challenge. Here, a reversible H+ /NH4 + co-insertion/extraction mechanism was developed in an aqueous Zn/MnO2 battery system. The synergistic effect between the dual cations endows the aqueous batteries with the fast kinetics of ion diffusion and the reversible structure evolution of MnO2 . As a result, the Zn/MnO2 battery displays excellent rate capability and cycling performance. This work will pave the way toward the design of aqueous rechargeable batteries with non-metallic ions.

Flexible all-in-one zinc-ion batteries.

The recent development of flexible and wearable electronic devices has increased the demand for energy storage devices with excellent flexibility and structural stability. Aqueous zinc-ion batteries (ZIBs) are promising energy storage devices due to their low cost, high safety, and eco-friendliness. Therefore, flexible ZIBs have to be considered. Herein, we design the flexible all-in-one ZIBs, where the reduced graphene oxide/polyaniline (rGO/PANI) cathode, cellulose nanofiber (CNF) separator, and exfoliated graphene (EG)/Zn anode are integrated together using an all-freeze-casting strategy. The continuous seamless connection of such all-in-one ZIBs can avoid displacing and detaching between the electrodes and separator under different bending states and improve the load-transfer capacity and interface strength between the neighboring component layers. As a result, the all-in-one ZIBs show excellent flexibility and superior electrochemical stability under different bending states.

A Self-Healing Integrated All-in-One Zinc-Ion Battery.

The self-healing of zinc-ion batteries (ZIBs) will not only significantly improve the durability and extend the lifetime of devices, but also decrease electronic waste and economic cost. A poly(vinyl alcohol)/zinc trifluoromethanesulfonate (PVA/Zn(CF3 SO3 )2 ) hydrogel electrolyte was fabricated by a facile freeze/thaw strategy. PVA/Zn(CF3 SO3 )2 hydrogels possess excellent ionic conductivity and stable electrochemical performance. Such hydrogel electrolytes can autonomously self-heal by hydrogen bonding without any external stimulus. All-in-one integrated ZIBs can be assembled by incorporating the cathode, separator, and anode into hydrogel matrix since the fabrication of PVA/Zn(CF3 SO3 )2 hydrogel is a process of converting the liquid to quasi-solid state. The ZIBs show an outstanding self-healing and can recover electrochemical performance completely even after several cutting/healing cycles.