Graphene faces the same problem, but its self-assembly into a 3D structures can effectively solve these problems. [5,6] The gelation of graphene oxide (GO) is the mostly used method to prepare a 3D assembled structure, and this process is triggered by the surface chemistry changes of GO NSs in solution. However, the gelation of the other 2D materials, such as transition metal dichalcogenides (TMDs), transition metal oxides (TMOs), and h-BN, [7][8][9][10] is difficult to achieve because of the fewer functional groups on their surface and their poor dispersion in an aqueous solution.Different from the above 2D materials, MXenes prepared by selective etching contain a number of functional groups on their surfaces and edges (F − , O, and OH) [11] show good dispersion in aqueous solution and have the potential to build 3D MXene-based assemblies by solution-based techniques. With the GO as the linker, 3D graphene/MXene hybrid monoliths have been assembled in solution with a low temperature heating. [12,13] In the assembly processes, the laminated MXene domains interacted with the π-conjugated GO sheets which then guided the formation of the 3D structure. The use of surfactants or polymers as the linkers to realize side-by-side joining of MXene NSs also has the potential to build 3D MXene-based monoliths. [14][15][16] However, all these strategies involve a complex process in which the MXenes are easily oxidized due to their high chemical activity. In addition, the above assembly process is different from the GO assembly that relies on self-gelation and has many advantages for achieving structure control by simply changing the solvent or the drying process. [17,18] Until now, the gelation of MXene in a similar manner to that of GO has been difficult to realize.Here, we report the fast gelation of MXene in an aqueous solution initiated by divalent metal ions. In a typical process, Fe 2+ ions are introduced in the MXene solution to destroy the electrostatic repulsive forces between the NSs and link them together, forming a stable 3D MXene hydrogel, similar to the GO hydrogel and its gelation process. [19] The use of metal ions as the joining sites enhances the interlinking of MXene NSs to form a 3D network due to their strong binding energy with the OH groups on the MXene surface. As a result, the formed hydrogel effectively suppresses the restacking of MXene NSs Gelation is an effective way to realize the self-assembly of nanomaterials into different macrostructures, and in a typical use, the gelation of graphene oxide (GO) produces various graphene-based carbon materials with different applications. However, the gelation of MXenes, another important type of 2D materials that have different surface chemistry from GO, is difficult to achieve. Here, the first gelation of MXenes in an aqueous dispersion that is initiated by divalent metal ions is reported, where the strong interaction between these ions and OH groups on the MXene surface plays a key role. Typically, Fe 2+ ions are introduced in the MXene dispersion whic...

Rechargeable aqueous zinc (Zn) ion‐based energy storage systems have been reviving recently because of their low cost and high safety merits; however, they still suffer from the problems of corrosion and dendrite growth on Zn metal anodes that cause gas generation and early battery failure. Unfortunately, the corrosion problem has not received sufficient attention until now. Here, it is pioneeringly demonstrated that decorating the Zn surface with a dual‐functional metallic indium (In) layer, acting as both a corrosion inhibitor and a nucleating agent, is a facile but effective strategy to suppress both drastic corrosion and dendrite growth. Symmetric cells assembled with the treated Zn electrodes can sustain up to 1500 h of plating/stripping cycles with an ultralow voltage hysteresis (54 mV), and a 5000 cycle‐life is achieved for a prototype full cell. This work will instigate the further development of aqueous metal‐based energy storage systems.

Accelerated conversion by catalysis is a promising way to inhibit shuttling of soluble polysulfides in lithium–sulfur (Li–S) batteries, but most of the reported catalysts work only for one direction sulfur reaction (reduction or oxidation), which is still not a root solution since fast cycled use of sulfur species is not finally realized. A bidirectional catalyst design, oxide–sulfide heterostructure, is proposed to accelerate both reduction of soluble polysulfides and oxidation of insoluble discharge products (e.g., Li2S), indicating a fundamental way for improving both the cycling stability and sulfur utilization. Typically, a TiO2–Ni3S2 heterostructure is prepared by in situ growing TiO2 nanoparticles on Ni3S2 surface and the intimately bonded interfaces are the key for bidirectional catalysis. For reduction, TiO2 traps while Ni3S2 catalytically converts polysulfides. For oxidation, TiO2 and Ni3S2 both show catalytic activity for Li2S dissolution, refreshing the catalyst surface. The produced sulfur cathode with TiO2–Ni3S2 delivers a low capacity decay of 0.038% per cycle for 900 cycles at 0.5C and specially, with a sulfur loading of 3.9 mg cm−2, achieves a high capacity retention of 65% over 500 cycles at 0.3C. This work unlocks how a bidirectional catalyst works for boosting Li–S batteries approaching practical uses.

A three-dimensional (3D) hierarchical porous graphene macrostructure coupled with uniformly distributed α-Fe 2 O 3 nano-particles (denoted Fe-PGM) was designed as a sulfur host in a Lithiumsulfur battery, and was prepared by a hydrothermal method. In this hybrid structure, the α-Fe 2 O 3 nanoparticles are proved to not only strongly interact with the polysulfides, but more importantly, chemically promote their transformation to insoluble species during the charge/discharge process, working as a chemical barrier for the shuttling of the lithium

MXenes can be denoted with the formula M n+1 X n T x (n 1-3), where M is an early transition metal (Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, or W), X is carbon and/or nitrogen, and T x stands for a surface functional group (O, OH, and/or F, etc.). [8] MXenes have a lattice with hexagonal lattice symmetry inherited from their parent MAX phase and have a superior electrical conductivity (6000-8000 S cm −1 ) due to their metallic backbone. [1,8] Abundant surface functional groups provide a large number of active sites on the surface of MXenes, and this has tremendous potential for surface modification and the highly efficient loading of active substances. [9][10][11][12] Additionally, MXenes have good thermal conductivity, [6] a tunable bandgap and excellent mechanical strength, [13,14] thus having great prospects in the fields of energy conversion and storage, [15][16][17] electromagnetic interference (EMI) shielding and absorption, [18][19][20] sensing, [21,22] environmental protection, and so on. [23,24] However, similar to other 2D materials, MXenes also have a propensity for "face to face" stacking and aggregation due to strong van der Waals forces, which greatly limits their performance in practical applications. [25,26] Recently, to address the stacking problem for a high surface utilization and obtain functional MXene materials with well-tailored structures, intense efforts from different aspects, such as surface modification, [27] heteroatom doping, [28] buckling and crumpling, [29][30][31][32] introducing interlayer spacers, [33] templating, [34] and crosslinking [35] have been made.Ways of assembling MXenes into 3D structures at the macro and/or microlevel are greatly needed to overcome the restacking problem of 2D materials, and have been extensively studied. [18,36,37] The 2D MXene nanosheets can be used as building blocks to construct MXene assemblies with desired structures by well-designed methods. Their excellent properties are expected to be inherited by the assemblies, thus essentially expanding their applications. Hence, there is a great need to summarize recent progress in the design of MXene assemblies for diverse applications.Previously, several reviews on the synthesis, properties, and applications of MXenes have been published. [8,15,25,[38][39][40][41] However, none of them have summarized the latest advances on MXene from an assembly perspective. A timely and focused progress report of MXene assemblies is expected to further accelerate the development of these emerging materials and promote their applications. Here, recent efforts on MXene assemblies and the corresponding assembly strategies are reviewed. To facilitate the discussion, the assemblies are classified into three categories according to their dimensional structures at the macro and/ or microlevels. These are, 2D assemblies, 2D macroassemblies Since their discovery in 2011, transition metal carbides or nitrides (MXenes) have attracted a wide range of attention due to their unique properties and promise for use in a variety of appl...

lems by the physical confinement with nanostructured carbon materials [4][5][6][7][8] and the chemical adsorption with various noncarbon oxides/sulfides/nitrides. [9][10][11][12][13][14][15][16] However, the weak affinity of carbonbased materials and the limited adsorption capacity of noncarbon materials toward polar LiPSs make these strategies fail to meet the high sulfur loading electrode and the long cycling process. The basic reason should be ascribed to the slow conversion of high concentrated LiPSs, where their accumulation in the electrolyte causes severe shuttling between electrodes.Recently, the catalysis in Li-S batteries has received much attention because the introduction of catalyst accelerates the LiPS conversion and then, fundamentally suppresses their shuttling even with high sulfur loading. [17] To accelerate the conversion of LiPSs, the catalytic materials should not only have strong adsorption ability toward LiPSs, but also the good conductivity and activity for their conversion. Besides, the relatively high surface area for the Li 2 S deposition is also required. Nevertheless, all these characters are hard to be integrated into one material. Our group previously proposed a TiO 2 -TiN heterostructure that combines the advantages of TiO 2 with the strong adsorption ability and TiN with excellent conductivity, and more importantly, forms abundant active interface to realize the smooth trapping-diffusion-conversion of LiPSs. [18] UntilThe lithium-sulfur (Li-S) battery is a next generation high energy density battery, but its practical application is hindered by the poor cycling stability derived from the severe shuttling of lithium polysulfides (LiPSs). Catalysis is a promising way to solve this problem, but the rational design of relevant catalysts is still hard to achieve. This paper reports the WS 2 -WO 3 heterostructures prepared by in situ sulfurization of WO 3 , and by controlling the sulfurization degree, the structure is controlled, which balances the trapping ability (by WO 3 ) and catalytic activity (by WS 2 ) toward LiPSs. As a result, the WS 2 -WO 3 heterostructures effectively accelerate LiPS conversion and improve sulfur utilization. The Li-S battery with 5 wt% WS 2 -WO 3 heterostructures as additives in the cathode shows an excellent rate performance and good cycling stability, revealing a 0.06% capacity decay each cycle over 500 cycles at 0.5 C. By building an interlayer with such heterostructure-added graphenes, the battery with a high sulfur loading of 5 mg cm −2 still shows a high capacity retention of 86.1% after 300 cycles at 0.5 C. This work provides a rational way to prepare the metal oxide-sulfide heterostructures with an optimized structure to enhance the performance of Li-S batteries.

No abstract

Supercapacitors are increasingly in demand among energy storage devices. Due to their abundant porosity and low cost, activated carbons are the most promising electrode materials and have been commercialized in supercapacitors for many years. However, their low packing density leads to an unsatisfactory volumetric performance, which is a big obstacle for their practical use where a high volumetric energy density is necessary. Inspired by the dense structure of irregular pomegranate grains, a simple yet effective approach to pack activated carbons into a compact graphene network with graphene as the “peels” is reported here. The capillary shrinkage of the graphene network sharply reduces the voids between the activated carbon particles through the microcosmic rearrangement while retaining their inner porosity. As a result, the electrode density increases from 0.41 to 0.76 g cm −3 . When used as additive‐free electrodes for supercapacitors in an ionic liquid electrolyte, this porous yet dense electrode delivers a volumetric capacitance of up to 138 F cm −3 , achieving high gravimetric and volumetric energy densities of 101 Wh kg −1 and 77 Wh L −1 , respectively. Such a graphene‐assisted densification strategy can be extended to the densification of other carbon or noncarbon particles for energy devices requiring a high volumetric performance.