2D MXene-based nanomaterials have attracted tremendous attention because of their unique physical/chemical properties and wide range of applications in energy storage, catalysis, electronics, optoelectronics, and photonics. However, MXenes and their derivatives have many inherent limitations in terms of energy storage applications. In order to further improve their performance for practical application, the nanoengineering of these 2D materials is extensively investigated. In this Review, the latest research and progress on 2D MXene-based nanostructures is introduced and discussed, focusing on their preparation methods, properties, and applications for energy storage such as lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, and supercapacitors. Finally, the critical challenges and perspectives required to be addressed for the future development of these 2D MXene-based materials for energy storage applications are presented.

Nanoengineering of 2D MXene-Based Materials for Energy Storage Applications

The development of sodium-ion batteries for large-scale applications requires the synthesis of electrode materials with high capacity, high initial Coulombic efficiency (ICE), high rate performance, long cycle life, and low cost. A rational design of freestanding anode materials is reported for sodium-ion batteries, consisting of molybdenum disulfide (MoS2) nanosheets aligned vertically on carbon paper derived from paper towel. The hierarchical structure enables sufficient electrode/electrolyte interaction and fast electron transportation. Meanwhile, the unique architecture can minimize the excessive interface between carbon and electrolyte, enabling high ICE. The as-prepared MoS2@carbon paper composites as freestanding electrodes for sodium-ion batteries can liberate the traditional electrode manufacturing procedure, thereby reducing the cost of sodium-ion batteries. The freestanding MoS2@carbon paper electrode exhibits a high reversible capacity, high ICE, good cycling performance, and excellent rate capability. By exploiting in situ Raman spectroscopy, the reversibility of the phase transition from 2H-MoS2 to 1T-MoS2 is observed during the sodium-ion intercalation/deintercalation process. This work is expected to inspire the development of advanced electrode materials for high-performance sodium-ion batteries.

MoS2 Nanosheets Vertically Aligned on Carbon Paper: A Freestanding Electrode for Highly Reversible Sodium Ion Batteries

Recent progress in the synthesis of graphene and derived materials for next generation electrodes of high performance lithium ion batteries

Metal-organic frameworks (MOFs), also quoted as porous coordination polymers (PCPs), are causing great concern in supercapacitors (SCs) field owing to their ultra-high surface-areas, tailorable pore-sizes and shapes, and diverse structural architectures. This review mainly focuses on the recent progress in various branches of MOFs materials including porous coordination networks, two-dimensional (2D) MOFs, entangled MOFs, polyoxometalate MOFs (POMOFs), heterometallic MOFs, and some new emerging MOFs, as well as their applications in SCs. The superiority and the deficiency of various MOF types were systematically introduced and summarized. Additionally, the challenges and perspectives relate to pristine MOFs and MOFs-based composites for the applications in SCs have also been discussed. We hope that our review could provide guiding frameworks to design and fabricate MOFs materials with more practical energy-storage applications.

Recent progress in metal-organic frameworks as active materials for supercapacitors

The rapid development of electrochemical energy storage (EES) systems requires novel electrode materials with high performance. A typical 2D nanomaterial, layered transition metal dichalcogenides (TMDs) are regarded as promising materials used for EES systems due to their large specific surface areas and layer structures benefiting fast ion transport. The typical methods for the preparation of TMDs and TMD-based nanohybrids are first summarized. Then, in order to improve the electrochemical performance of various kinds of rechargeable batteries, such as lithium-ion batteries, lithium-sulfur batteries, sodium-ion batteries, and other types of emerging batteries, the strategies for the design and fabrication of layered TMD-based electrode materials are discussed. Furthermore, the applications of layered TMD-based nanomaterials in supercapacitors, especially in untraditional supercapacitors, are presented. Finally, the existing challenges and promising future research directions in this field are proposed.

Layered Transition Metal Dichalcogenide-Based Nanomaterials for Electrochemical Energy Storage.

Hierarchical Hollow‐Microsphere Metal–Selenide@Carbon Composites with Rational Surface Engineering for Advanced Sodium Storage

Garnet Solid Electrolyte for Advanced All-Solid-State Li Batteries

Sodium-ion capacitors (SICs), as new-generation electrochemical energy-storage systems, have combined the advantages of high energy and power densities, meeting the urgent demand for versatile electronic equipment and grid energy-storage stations. Nevertheless, the electrochemical performance of SICs is seriously restricted by the kinetic mismatch between the battery-type anode and capacitor-type cathode. In this work, N-doped 3D carbon (NHPC) delivered a high reversible specific capacity of 197 mA h g−1 at 2 A g−1, and the mechanism of its electrochemistry mainly involved strong pseudocapacitive storage that promoted quick physical adsorption/desorption. Moreover, further activation of NHPC yielded nitrogen-doped hierarchical porous activated carbon (NHPAC), which displayed a large specific surface area of 1478 m2 g−1 with abundant meso/macropores, and brought about fast adsorption/desorption of anions on its surface. The full-carbon SIC device benefitted from the similar material systems used for its anode and cathode, and hence achieved a high energy density of 115 W h kg−1 at 200 W kg−1 as well as long-term cyclability in the potential range of 0–4.0 V. This rational strategy of kinetic matching might open up a potential avenue for the development of additional advanced SICs.

A kinetically well-matched full-carbon sodium-ion capacitor

Layered material MoS2 is widely applied as a promising anode for lithium-ion batteries (LIBs). Herein, a scalable and facile dopamine-assisted hydrothermal technique for the preparation of strongly coupled MoS2 nanosheets and nitrogen-doped graphene (MoS2 /N-G) composite is developed. In this composite, the interconnected MoS2 nanosheets are well wrapped onto the surface of graphene, forming a unique veil-like architecture. Experimental results indicate that dopamine plays multiple roles in the synthesis: a binding agent to anchor and uniformly disperse MoS2 nanosheets, a morphology promoter, and the precursor for in situ nitrogen doping during the self-polymerization process. Density functional theory calculations further reveal that a strong interaction exists at the interface of MoS2 nanosheets and nitrogen-doped graphene, which facilitates the charge transfer in the hybrid system. When used as the anode for LIBs, the resulting MoS2 /N-G composite electrode exhibits much higher and more stable Li-ion storage capacity (e.g., 1102 mAh g-1 at 100 mA g-1 ) than that of MoS2 /G electrode without employing the dopamine linker. Significantly, it is also identified that the thin MoS2 nanosheets display outstanding high-rate capability due to surface-dominated pseudocapacitance contribution.

Strong Coupling of MoS2 Nanosheets and Nitrogen-Doped Graphene for High-Performance Pseudocapacitance Lithium Storage

The practical implementation of Zn metal anodes with high volumetric capacity is seriously plagued by the dendritic growth and accompanying interfacial parasitic reactions. Herein, high yield carbon dots (CDs) with abundant polar functional groups (CHO and CN), as a functional artificial interface layer, are rationally designed to optimize electrolyte/Zn interfaces with large‐scale viability. Of particular note, the quantum‐sized CDs with strong Zn affinity can effectively ameliorate the electric field distribution and ensure that more Zn2+ is adsorbed onto the whole electrode, which are beneficial for lowering the barrier of Zn2+ nucleation and inducing homogeneous Zn deposition, thus rendering a dendrite‐free Zn anode, as extensively confirmed by in situ optical microscope observation and finite element simulation. Meanwhile, the dense and insoluble coating layer with abundant polar functional groups is conducive to arousing the repulsion effect, which is good for shielding the active water and SO42−, thus eliminating the water‐mediated parasitic reactions and improving Zn2+ reaction kinetics. More importantly, the electrochemically stable CDs layer endows the Zn anode with a prolonged lifespan of 3000 h at 1 mA cm−2. This feasible and efficient fabrication of functional CDs layer opens a new avenue for stable dendrite‐free metal anodes.

Laiqiang Xu

Papers

Hierarchical Hollow‐Microsphere Metal–Selenide@Carbon Composites with Rational Surface Engineering for Advanced Sodium Storage

Garnet Solid Electrolyte for Advanced All-Solid-State Li Batteries

A kinetically well-matched full-carbon sodium-ion capacitor

Strong Coupling of MoS2 Nanosheets and Nitrogen-Doped Graphene for High-Performance Pseudocapacitance Lithium Storage

High‐Yield Carbon Dots Interlayer for Ultra‐Stable Zinc Batteries