Although current high-energy-density lithium-ion batteries (LIBs) have taken over the commercial rechargeable battery market, increasing concerns about limited lithium resources, high cost, and insecurity of organic electrolyte scale-up limit their further development. Rechargeable aqueous zinc-ion batteries (ZIBs), an alternative battery chemistry, have paved the way not only for realizing environmentally benign and safe energy storage devices but also for reducing the manufacturing costs of next-generation batteries. This Review underscores recent advances in aqueous ZIBs; these include the design of a highly reversible Zn anode, optimization of the electrolyte, and a wide range of cathode materials and their energy storage mechanisms. We also present recent advanced techniques that aim at overcoming the current issues in aqueous ZIB systems. This Review on the future perspectives and research directions will provide a guide for future aqueous ZIB study.

Recent Advances in Aqueous Zinc-Ion Batteries

Zinc-ion batteries built on water-based electrolytes featuring compelling price-points, competitive performance, and enhanced safety represent advanced energy storage chemistry as a promising alternative to current lithium-ion battery systems. Attempts to develop rechargeable aqueous zinc-ion batteries (ZIBs) can be traced to as early as the 1980s; however, since 2015, the research activity in this field has surged throughout the world. Despite the achievements made in exploring electrode materials so far, significant challenges remain at the material level and even on the whole aqueous ZIBs system, leading to the failure of ZIBs to meet commercial requirements. This review aims to discuss how to pave the way for developing aqueous ZIBs. The current research efforts related to aqueous ZIBs electrode materials and electrolytes are summarized, including an analysis of the problems encountered in both cathode/anode materials and electrolyte optimization. Some concerns and feasible solutions for achieving practical aqueous ZIBs are discussed in detail. We would like to point out that merely improving the electrode materials is not enough; synergistic optimization strategies toward the whole battery system are also deeply needed. Finally, some perspectives are provided on the subsequent optimization design for further research efforts in the aqueous ZIB field.

Issues and opportunities facing aqueous zinc-ion batteries

Rechargeable aqueous zinc-ion batteries are promising energy storage devices due to their high safety and low cost. However, they remain in their infancy because of the limited choice of positive electrodes with high capacity and satisfactory cycling performance. Furthermore, their energy storage mechanisms are not well established yet. Here we report a highly reversible zinc/sodium vanadate system, where sodium vanadate hydrate nanobelts serve as positive electrode and zinc sulfate aqueous solution with sodium sulfate additive is used as electrolyte. Different from conventional energy release/storage in zinc-ion batteries with only zinc-ion insertion/extraction, zinc/sodium vanadate hydrate batteries possess a simultaneous proton, and zinc-ion insertion/extraction process that is mainly responsible for their excellent performance, such as a high reversible capacity of 380 mAh g–1 and capacity retention of 82% over 1000 cycles. Moreover, the quasi-solid-state zinc/sodium vanadate hydrate battery is also a good candidate for flexible energy storage device. Rechargeable zinc-ion batteries are promising energy storage devices but suffer from the limited choice of positive electrodes. Here Niu and co-workers show a design with sodium vanadate hydrate as cathode, allowing simultaneous proton and zinc-ion insertion/extraction and enhanced performance.

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Aqueous rechargeable zinc/sodium vanadate batteries with enhanced performance from simultaneous insertion of dual carriers.

https://www.cell.com/joule/pdf/S2542-4351(20)30094-5.pdf

Scientific Challenges for the Implementation of Zn-Ion Batteries

Safety concerns about organic media-based batteries are the key public arguments against their widespread usage. Aqueous batteries (ABs), based on water which is environmentally benign, provide a promising alternative for safe, cost-effective, and scalable energy storage, with high power density and tolerance against mishandling. Research interests and achievements in ABs have surged globally in the past 5 years. However, their large-scale application is plagued by the limited output voltage and inadequate energy density. We present the challenges in AB fundamental research, focusing on the design of advanced materials and practical applications of whole devices. Potential interactions of the challenges in different AB systems are established. A critical appraisal of recent advances in ABs is presented for addressing the key issues, with special emphasis on the connection between advanced materials and emerging electrochemistry. Last, we provide a roadmap starting with material design and ending with the commercialization of next-generation reliable ABs.

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Roadmap for advanced aqueous batteries: From design of materials to applications.

The current boom of safe and renewable energy storage systems is driving the recent renaissance of Zn-ion batteries. However, the notorious tip-induced dendrite growth on the Zn anode restricts their further application. Herein, the first demonstration of constructing a flexible 3D carbon nanotube (CNT) framework as a Zn plating/stripping scaffold is constituted to achieve a dendrite-free robust Zn anode. Compared with the pristine deposited Zn electrode, the as-fabricated Zn/CNT anode affords lower Zn nucleation overpotential and more homogeneously distributed electric field, thus being more favorable for highly reversible Zn plating/stripping with satisfactory Coulombic efficiency rather than the formation of Zn dendrites or other byproducts. As a consequence, a highly flexible symmetric cell based on the Zn/CNT anode presents appreciably low voltage hysteresis (27 mV) and superior cycling stability (200 h) with dendrite-free morphology at 2 mA cm-2 , accompanied by a high depth of discharge (DOD) of 28%. Such distinct performance overmatches most of recently reported Zn-based anodes. Additionally, this efficient rechargeability of the Zn/CNT anode also enables a substantially stable Zn//MnO2 battery with 88.7% capacity retention after 1000 cycles and remarkable mechanical flexibility.

Dendrite‐Free Zinc Deposition Induced by Multifunctional CNT Frameworks for Stable Flexible Zn‐Ion Batteries

Advanced flexible batteries with high energy density and long cycle life are an important research target. Herein, the first paradigm of a high-performance and stable flexible rechargeable quasi-solid-state Zn–Mn2 battery is constructed by engineering MnO2 electrodes and gel electrolyte. Benefiting from a poly(3,4-ethylenedioxythiophene) (PEDOT) buffer layer and a Mn2+-based neutral electrolyte, the fabricated Zn–Mn2@PEDOT battery presents a remarkable capacity of 366.6 mA h g−1 and good cycling performance (83.7% after 300 cycles) in aqueous electrolyte. More importantly, when using PVA/ZnCl2/MnSO4 gel as electrolyte, the as-fabricated quasi-solid-state Zn–Mn2@PEDOT battery remains highly rechargeable, maintaining more than 77.7% of its initial capacity and nearly 100% Coulombic efficiency after 300 cycles. Moreover, this flexible quasi-solid-state Zn–Mn2 battery achieves an admirable energy density of 504.9 W h kg−1 (33.95 mW h cm−3), together with a peak power density of 8.6 kW kg−1, substantially higher than most recently reported flexible energy-storage devices. With the merits of impressive energy density and durability, this highly flexible rechargeable Zn–Mn2 battery opens new opportunities for powering portable and wearable electronics.

Achieving Ultrahigh Energy Density and Long Durability in a Flexible Rechargeable Quasi-Solid-State Zn-MnO2 Battery.

Currently, the main bottleneck for the widespread application of Ni–Zn batteries is their poor cycling stability as a result of the irreversibility of the Ni-based cathode and dendrite formation of the Zn anode during the charging–discharging processes. Herein, a highly rechargeable, flexible, fiber-shaped Ni–Zn battery with impressive electrochemical performance is rationally demonstrated by employing Ni–NiO heterostructured nanosheets as the cathode. Benefiting from the improved conductivity and enhanced electroactivity of the Ni–NiO heterojunction nanosheet cathode, the as-fabricated fiber-shaped Ni–NiO//Zn battery displays high capacity and admirable rate capability. More importantly, this Ni–NiO//Zn battery shows unprecedented cyclic durability both in aqueous (96.6% capacity retention after 10 000 cycles) and polymer (almost no capacity attenuation after 10 000 cycles at 22.2 A g−1) electrolytes. Moreover, a peak energy density of 6.6 µWh cm−2, together with a remarkable power density of 20.2 mW cm−2, is achieved by the flexible quasi-solid-state fiber-shaped Ni–NiO//Zn battery, outperforming most reported fiber-shaped energy-storage devices. Such a novel concept of a fiber-shaped Ni–Zn battery with impressive stability will greatly enrich the flexible energy-storage technologies for future portable/wearable electronic applications.

An Ultrastable and High-Performance Flexible Fiber-Shaped Ni-Zn Battery based on a Ni-NiO Heterostructured Nanosheet Cathode.

The poor intrinsic conductivity of MoS2 presents a huge barrier for the exploitation of its versatile properties, especially as an electrochemical capacitor (EC) electrode and hydrogen evolution reaction (HER) catalyst. Toward this challenge, TiN nanorods coated by randomly oriented MoS2 nanosheets (TMSs) are engineered as state-of-the-art electrodes for ECs and HER. In light of the synergistic effects, TMS electrodes show favorable performance as both a binder-free EC electrode and HER catalyst. Importantly, the optimal TMS achieves an areal capacitance of 662.2 mF cm–2 at 1 mA cm–2 with superior rate capability and ultralong cycling stability. As the catalyst for HER in 0.5 M H2SO4, it shows an overpotential of 146 mV at 10 mA cm–2, a favorable Tafel slope, and good electrocatalytic stability. All of the results highlight the favorable integration of TiN and MoS2 and provide clear insight correlating the hybrid structure and the corresponding electrochemical performance.

Xiyue Zhang

Papers

Dendrite‐Free Zinc Deposition Induced by Multifunctional CNT Frameworks for Stable Flexible Zn‐Ion Batteries

Achieving Ultrahigh Energy Density and Long Durability in a Flexible Rechargeable Quasi-Solid-State Zn-MnO2 Battery.

An Ultrastable and High-Performance Flexible Fiber-Shaped Ni-Zn Battery based on a Ni-NiO Heterostructured Nanosheet Cathode.

Engineering Thin MoS2 Nanosheets on TiN Nanorods: Advanced Electrochemical Capacitor Electrode and Hydrogen Evolution Electrocatalyst

Oxygen Defects in Promoting the Electrochemical Performance of Metal Oxides for Supercapacitors: Recent Advances and Challenges