Transition‐metal (Fe, Co, Ni) based metal‐organic framework materials with controllable structures, large surface areas and adjustable pore sizes have attracted wide research interest for use in next‐generation electrochemical energy‐storage devices. This review introduces the synthesis of transition‐metal (Fe, Co, Ni) based metal‐organic frameworks and their derivatives with the focus on their application in supercapacitors and batteries.

Rechargeable zinc–air batteries show great potential in applications such as electric vehicles and wearable devices, especially for the flexible design. And the challenges and functional materials for each component are provided and discussed from air electrode, solid-state electrolyte to zinc anode, with perspectives of research directions.

factors: electrode materials, electrolytes, and separators. [18] Finding suitable electrode materials to promote and realize their commercialization is still considered one of the major challenges. [19][20][21] Up to now, electrode materials have commonly included carbon materials (CMs), [5,13,22] conducting polymer materials (CPMs), [23] transition metal oxides (TMOs), [24] and others. [25][26][27] However, they all have their respective shortcomings; for instance, CMs tend to restack and then decrease the specific surface area, which is an important index for CM performance. [28][29][30] CPMs show the same phenomenon that the original structure will collapse under long-term charge/discharge process. [23] Although the above two types of electrode materials have good electron conductivity, their changing specific surface area and structures during use make them unlikely to be promising electrode materials. TMOs always have a good reversible faradic reaction due to their valence flexibility. However, in comparison to CMs, their poor electron conductivity may lower their specific capacity. [31][32][33] Transition metal sulfides (TMSs) have attracted tremendous attention due to their high specific capacity. For example, nickel and cobalt sulfides (e.g., NiS x , CoS x ) have specific capacities that are double of their oxide counterparts (e.g., NiO x , CoO x ). [22,[34][35][36] This is because the replacement of oxygen with sulfur, an element with a lower electronegativity, increases the performance compared to TMOs. [37,38] However, their unyielding volume change during the cycling process has hindered their further development and application in lithium and sodium rechargeable batteries. [39] Though TMSs possess high specific capacities and excellent rate capabilities when used for SCs, it is difficult to achieve all these objectives simultaneously from a single material. Therefore, the hybridization of different materials with different properties is becoming an interesting research area. Recent papers have testified that the adulteration of graphene or graphene derivatives can solve these issues and increase the electrochemical performance of energy storage devices. [40][41][42] It can be attributed to the properties of graphene: 2D conductive networks, a large specific surface area, and good physicochemical stability. In addition to these properties, their porous structure can effectively promote the diffusion of electrolyte ions. Therefore, 2D graphene is one of the ideal support framework materials to prepare TMS@graphene composites for electrode materials. [43][44][45][46] Transition metal sulfides, as an important class of inorganics, can be used as excellent electrode materials for various types of electrochemical energy storage, such as lithium-ion batteries, sodium-ion batteries, supercapacitors, and others. Recent works have identified that mixing graphene or graphene derivatives with transition metal sulfides can result in novel composites with better electrochemical performance. This review summarizes ...

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Lithium‐ion batteries (LIBs) have been widely used in the field of portable electric devices because of their high energy density and long cycling life. To further improve the performance of LIBs, it is of great importance to develop new electrode materials. Various transition metal oxides (TMOs) have been extensively investigated as electrode materials for LIBs. According to the reaction mechanism, there are mainly two kinds of TMOs, one is based on conversion reaction and the other is based on intercalation/deintercalation reaction. Recently, hierarchically nanostructured TMOs have become a hot research area in the field of LIBs. Hierarchical architecture can provide numerous accessible electroactive sites for redox reactions, shorten the diffusion distance of Li‐ion during the reaction, and accommodate volume expansion during cycling. With rapid research progress in this field, a timely account of this advanced technology is highly necessary. Here, the research progress on the synthesis methods, morphological characteristics, and electrochemical performances of hierarchically nanostructured TMOs for LIBs is summarized and discussed. Some relevant prospects are also proposed.

Typical layered transition‐metal chalcogenide materials, in particular layered molybdenum disulfide (MoS2) nanocomposites, have attracted increasing attention in recent years due to their excellent chemical and physical properties in various research fieldsHere, a general overview of synthetic MoS2 based nanocomposites via different preparation approaches and their applications in energy storage devices (Li‐ion battery, Na‐ion battery, and supercapacitor) is presented. The relationship between morphologies and the electrochemical performances of MoS2‐based nanocomposites in the three typical and promising rechargeable systems is also discussed. Finally, perspectives on major challenges and opportunities faced by MoS2‐based materials to address the practical problems of MoS2‐based materials are presented.

2D materials are ideal for constructing flexible electrochemical energy storage devices due to their great advantages of flexibility, thinness, and transparency. Here, a simple one‐step hydrothermal process is proposed for the synthesis of nickel–cobalt phosphate 2D nanosheets, and the structural influence on the pseudocapacitive performance of the obtained nickel–cobalt phosphate is investigated via electrochemical measurement. It is found that the ultrathin nickel–cobalt phosphate 2D nanosheets with an Ni/Co ratio of 4:5 show the best electrochemical performance for energy storage, and the maximum specific capacitance up to 1132.5 F g−1. More importantly, an aqueous and solid‐state flexible electrochemical energy storage device has been assembled. The aqueous device shows a high energy density of 32.5 Wh kg−1 at a power density of 0.6 kW kg−1, and the solid‐state device shows a high energy density of 35.8 Wh kg−1 at a power density of 0.7 kW kg−1. These excellent performances confirm that the nickel–cobalt phosphate 2D nanosheets are promising materials for applications in electrochemical energy storage devices.

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