Energy production and storage technologies have attracted a great deal of attention for day-to-day applications. In recent decades, advances in lithium-ion battery (LIB) technology have improved living conditions around the globe. LIBs are used in most mobile electronic devices as well as in zero-emission electronic vehicles. However, there are increasing concerns regarding load leveling of renewable energy sources and the smart grid as well as the sustainability of lithium sources due to their limited availability and consequent expected price increase. Therefore, whether LIBs alone can satisfy the rising demand for small- and/or mid-to-large-format energy storage applications remains unclear. To mitigate these issues, recent research has focused on alternative energy storage systems. Sodium-ion batteries (SIBs) are considered as the best candidate power sources because sodium is widely available and exhibits similar chemistry to that of LIBs; therefore, SIBs are promising next-generation alternatives. Recently, sodiated layer transition metal oxides, phosphates and organic compounds have been introduced as cathode materials for SIBs. Simultaneously, recent developments have been facilitated by the use of select carbonaceous materials, transition metal oxides (or sulfides), and intermetallic and organic compounds as anodes for SIBs. Apart from electrode materials, suitable electrolytes, additives, and binders are equally important for the development of practical SIBs. Despite developments in electrode materials and other components, there remain several challenges, including cell design and electrode balancing, in the application of sodium ion cells. In this article, we summarize and discuss current research on materials and propose future directions for SIBs. This will provide important insights into scientific and practical issues in the development of SIBs.

/pdf/sodium-ion-batteries-present-and-future-gf1bd0vhq4.pdf

Sodium-ion batteries: present and future

/pdf/recent-advances-in-zn-ion-batteries-utvauxy3ub.pdf

Recent Advances in Zn-Ion Batteries

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.

/pdf/aqueous-rechargeable-zinc-sodium-vanadate-batteries-with-42u45gzo5a.pdf

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

DOI: 10.1002/aenm.201601920 class of materials show great potential for the insertion/extraction of multivalent ions (Zn2+, Mg2+, Al3+) owing to the characteristic of large layer spacing and high conductivity. Among all the TMDs, VS2 is a typical family member of TMDs with hexagonal system, which shows similar crystal structure to that of graphite lamellar with an interlayer spacing of 5.76 Å.[25,30] There is a vanadium layer between two sulfur layers to form a kind of sandwich structure. In VS2 crystal structure, each V atom is arranged around six S atoms and connected with S atoms with covalent bonds. The interlayer spacing of VS2 is so large that enables the convenient insertion/extraction of lithium ions (0.69 Å), sodium ions (1.02 Å), zinc ions (0.74 Å) or their solvation sheath in electrolyte. However, to the best of our knowledge, there is no report about VS2 as the electrode materials for ZIBs. Herein, the VS2 nanosheets are synthesized via a facile hydrothermal reaction (Supporting Information), which deliver a high capacity of 190.3 mA h g−1 at a current density of 0.05 A g−1 and exhibit long-term cyclic stability as the cathode for ZIBs. The electrochemical reaction mechanism of such VS2 electrodes is further investigated systematically through a series of measurements including ex situ X-ray diffraction (XRD), ex situ X-ray photoelectron spectroscopy (XPS), in situ Raman, ex situ transmission electron microscopy (TEM). A reversible insertion/extraction process can be observed from all aspects. Both the ex situ TEM and ex situ XRD results demonstrate that the interlayer space of VS2 can self adapt to the intercalation of Zn2+ with an expansion along the c-axis (only 1.73%) and a slightly shrink along the aand b-axes, which plays a key role in the realization of long-life ZIBs. All the above evidences reveal that the VS2 is a promising cathode material with high capacity and good cyclic stability for ZIBs. The crystal structure of the as-prepared VS2 is tested by XRD. All characteristic peaks are in accordance with the standard card of VS2 (JCPDS NO. 01-089-1640) (Figure 1a). The Raman spectrum of the VS2 in the range of 100–1100 cm−1 is shown in Figure 1b. Six peaks located at 140.4, 192.0, 282.0, 406.6, 687.8, and 993.2 cm−1 are observed, which correspond to the rocking and stretching vibrations of V–S bonds or their combination.[25] The morphology and microstructures of as-prepared VS2 are investigated by field emission scanning electron microscopy (SEM) and high-resolution TEM (HRTEM). As shown in Figure 1c, The VS2 flowers are assembled by nanosheets with a diameter of 5–8 μm and a thickness of 50–100 nm. The d-spacing calculated from selected area electron diffraction (SAED) patterns are 2.89 and 1.64 Å (Figure 2f), which match the d-spacing values of (002) and (110) crystal planes of VS2, respectively. TEM and corresponding HRTEM images in Figure 2e show VS2 nanosheets with a d-spacing of ≈5.76 Å, The continuous researches of energy-storage devices have gained considerable attention in our world which results from the increased development of new-type energy caused by energy crisis and environmental pollution.[1–3] In the past several decades, lithium ion batteries have been widely explored and applied to various fields as they deliver higher energy density compared to other secondary batteries.[4,5] Nevertheless, the processing cost, complicated issues of safety, the limited lithium resources as well as some environmental issues lead to an urgent challenge for exploring new energy storage system.[6,7] The rechargeable aqueous batteries, such as aqueous sodiumion batteries and aqueous Zn ion batteries (ZIBs) have received incremental attention because of cost effectiveness and material abundance.[8–16] There is interest in aqueous ZIBs due to the safety, low cost, abundance of Zn source, and utilizing divalent cations to increase charge-storage capabilities. However, existing aqueous ZIBs are far from achieving the goals of excellent performances demanded by the ever increasing energy consumption. It’s hard to find cathode materials suitable for the reversible intercalation/deintercalation of Zn ions (or their solvation sheath in electrolyte), which limits the developmen of ZIBs.[16] The previous explorations of the cathode material mostly focus on manganese dioxide (MnO2) and Prussian blue analogues, whereas, the former suffers a poor rate performance and a rapid capacity fading, while the latter delivers limited capacities (about 50 mA h g−1).[17–22] Recently, Nazar and co-workers reported a high-capacity and long-life aqueous rechargeable zinc battery, composing of a Zn0.25V2O5⋅nH2O nanobelts cathode, 1 m ZnSO4 electrolyte, and a zinc anode.[23] The work indicates that the layered structure materials show great potential for the cathode of ZIBs. During the past decades, layered transition-metal dichalcogenides (TMDs), such as MoS2, WS2, and VS2 have received significant attentions in a variety of fields for their outstanding characteristic (graphene-like layered structure, direct bandgap, and fast ion diffusion).[24–26] These properties make TMDs potential candidates for battery electrode materials. When applied as the electrode materials for lithium/sodium ion battery, some excellent studies have been reported.[27–29] Also, this

Layered VS2 Nanosheet‐Based Aqueous Zn Ion Battery Cathode

Aqueous zinc-ion batteries attract increasing attention due to their low cost, high safety, and potential application in stationary energy storage. However, the simultaneous realization of high cycling stability and high energy density remains a major challenge. To tackle the above-mentioned challenge, we develop a novel Zn/V2O5 rechargeable aqueous hybrid-ion battery system by using porous V2O5 as the cathode and metallic zinc as the anode. The V2O5 cathode delivers a high discharge capacity of 238 mAh g–1 at 50 mA g–1. 80% of the initial discharge capacity can be retained after 2000 cycles at a high current density of 2000 mA g–1. Meanwhile, the application of a “water-in-salt” electrolyte results in the increase of discharge platform from 0.6 to 1.0 V. This work provides an effective strategy to simultaneously enhance the energy density and cycling stability of aqueous zinc ion-based batteries.

Zn/V2O5 Aqueous Hybrid-Ion Battery with High Voltage Platform and Long Cycle Life.

The aqueous zinc ion batteries (ZIBs) composed of inexpensive zinc anode and nontoxic aqueous electrolyte are attractive candidates for large-scale energy storage applications. However, their development is limited by cathode materials, which often deliver inferior rate capability and restricted cycle life. Herein, the VO2 nanorods show significant electrochemical performance when used as an intercalation cathode for aqueous ZIBs. Specifically, the VO2 nanorods display high initial capacity of 325.6 mAh g–1 at 0.05 A g–1, good rate capability, and excellent cycling stability of 5000 cycles at 3.0 A g–1. Furthermore, the VO2 unit cell expands in a, b, and c directions sequentially during the discharge process and contracts back reversibly during the charge process, and the zinc storage mechanism is revealed to be a highly reversible single-phase reaction by operando techniques and corresponding qualitative analyses. Our work not only opens a new door to the practical application of VO2 in ZIB systems but a...

Ultrastable and High-Performance Zn/VO2 Battery Based on a Reversible Single-Phase Reaction

DOI: 10.1002/aenm.201901469 further development.[2] Hence, exploring novel approaches to achieve more efficient energy storage is highly demanded. Recently, aqueous batteries are attracting unprecedented attention particularly owing to their high safety, high ion conductivity, low cost, and environmental friendliness.[3] To date, numerous aqueous batteries based on Li+, Na+, K+, Mg2+, Ca2+, Zn2+, Al3+, Fe3+, and/or mixed metal ions as charge carriers have been reported,[4] which find potential applications in fields such as grid-scale energy storage, wearable devices, and etc.[5] Among them, as a promising candidate, the rechargeable aqueous Zn-based batteries (AZBs) including Zn-ion batteries (mild electrolyte),[6] Zn–Co/Ag/ Ni alkaline batteries[7] and Zn–air batteries in alkaline electrolyte[8] have been extensively studied due to their unparalleled advantages of Zn anode. In general, metal Zn has the features of high theoretical capacity (820 mAh g−1), high electrical conductivity, nontoxicity, easy processing, and suitable redox potential (−0.76 V vs standard hydrogen electrode).[9] However, most of AZBs reported so far have encountered the same challenges, which are the narrow voltage window, unsatisfactory capacity, and poor cycling performance.[10] For example, all Zn-ion batteries operated in mild electrolyte including Zn//V-based, Zn//Mn-based, and Zn//Prussian blue analogs-based hold a narrow voltage window of 0.3–1.6, 0.9–1.8, and 0.2–1.8 V, respectively.[11] Even though AZBs in alkaline electrolyte display a higher voltage than that achieved in mild medium, their voltage windows are still only about 1.2–1.9 V.[12] Meanwhile, the alkaline electrolytes show stronger corrosion than mild neutral electrolytes, which greatly limit their wide applications. Moreover, the unstable cycling performance in AZBs due to the Zn dendrites and side reaction on the surface of Zn anode is also unsatisfactory.[10] To date, the electrolyte optimization or structural design are the common ways to suppress the growth of Zn dendrite and improve the cycling stability. For example, Chen and co-workers reported that aqueous electrolyte Zn(CF3SO3)2 can suppress the formation of detrimental dendrites in AZBs owing to the better reversibility and faster kinetics of Zn deposition/dissolution than that in ZnSO4 electrolyte.[13] However, Zn(CF3SO3)2 is too expensive (≈$ 8.1 g−1, prices from Sigma-Aldrich) to be applied With the increasing energy crisis and environmental pollution, rechargeable aqueous Zn-based batteries (AZBs) are receiving unprecedented attention due to their list of merits, such as low cost, high safety, and nontoxicity. However, the limited voltage window, Zn dendrites, and relatively low specific capacity are still great challenges. In this work, a new reaction mechanism of reversible Mn2+ ion oxidation deposition is introduced to AZBs. The assembled Mn2+/Zn2+ hybrid battery (Mn2+/Zn2+ HB) based on a hybrid storage mechanism including Mn2+ ion deposition, Zn2+ ion insertion, and conversion reaction of MnO2 can achieve an ultrawide voltage window (0–2.3 V) and high capacity (0.96 mAh cm−2). Furthermore, the carbon nanotubes coated Zn anode is proved to effectively inhibit Zn dendrites and control side reaction, hence exhibiting an ultrastable cycling (33 times longer than bare Zn foil) without obvious polarization. Benefiting from the optimal Zn anode and highly reversible Mn2+/Zn2+ hybrid storage mechanism, the Mn2+/Zn2+ HB shows an excellent cycling performance over 11 000 cycles with a 100% capacity retention. To the best of the authors’ knowledge, it is the highest reported cycling performance and wide voltage window for AZBs with mild electrolyte, which may inspire a great insight into designing high-performance aqueous batteries.

Lineng Chen

Papers

Layered VS2 Nanosheet‐Based Aqueous Zn Ion Battery Cathode

Zn/V2O5 Aqueous Hybrid-Ion Battery with High Voltage Platform and Long Cycle Life.

Ultrastable and High-Performance Zn/VO2 Battery Based on a Reversible Single-Phase Reaction

A Novel Dendrite‐Free Mn2+/Zn2+ Hybrid Battery with 2.3 V Voltage Window and 11000‐Cycle Lifespan

Recent Advances and Prospects of Cathode Materials for Rechargeable Aqueous Zinc‐Ion Batteries