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Vanadium diselenide (VSe2) is a transition metal dichalcogenide with metallic conductivity, which makes it a potentially promising electrode material for electrochemical applications. However, the development of VSe2electrodes for such applications has been severely hampered by the difficulty of preparing nanosized products. In this work, a new facile solvothermal synthesis process is developed and optimized to synthesize ultrathin VSe2nanosheet assemblies. To obtain the ultrathin nanosheets, N-methyl pyrrolidone, which has similar surface energy to many transition metal dichalcogenides, was used as the solvent to limit the crystal growth along the c-axis direction. The resulting ultrathin VSe2nanosheets exhibit good performance toward alkaline ion (Li+and Na+) storage, which can be significantly enhanced by carbon coating. Specifically, the carbon-coated VSe2nanosheets can deliver high capacities of 768 mA h g-1(Li+storage) and 571 mA h g-1(Na+storage) along with outstanding stability.

We report a phosphine (PH3) plasma activation strategy for significantly boosting the electrochemical performance of supercapacitor electrodes. Using Fe2O3as a demonstration, we show that the plasma activation simultaneously improves the conductivity, creates atomic-scale vacancies (defects), as well as increases active surface area, and thus leading to a greatly enhanced performance with a high areal capacitance of 340 mF cm−2at 1 mA cm−2, compared to 66 mF cm−2of pristine Fe2O3. Moreover, the asymmetric supercapacitor devices based on plasma-activated Fe2O3anodes and electrodeposited MnO2cathodes can achieve a high stack energy density of 0.42 mW h cm−3at a stack power density of 10.3 mW cm−3along with good stability (88% capacitance retention after 9000 cycles at 10 mA cm−2). Our work provides a simple yet effective strategy to greatly enhance the electrochemical performance of Fe2O3anodes and to further promote their application in asymmetric supercapacitors.

In this report, we show that oxide battery anodes can be grown on two-dimensional titanium carbide sheets (MXenes) by atomic layer deposition. Using this approach, we have fabricated a composite SnO2/MXene anode for Li-ion battery applications. The SnO2/MXene anode exploits the high Li-ion capacity offered by SnO2, while maintaining the structural and mechanical integrity of the conductive MXene platform. The atomic layer deposition (ALD) conditions used to deposit SnO2on MXene terminated with oxygen, fluorine, and hydroxyl-groups were found to be critical for preventing MXene degradation during ALD. We demonstrate that SnO2/MXene electrodes exhibit excellent electrochemical performance as Li-ion battery anodes, where conductive MXene sheets act to buffer the volume changes associated with lithiation and delithiation of SnO2. The cyclic performance of the anodes is further improved by depositing a very thin passivation layer of HfO2, in the same ALD reactor, on the SnO2/MXene anode. This is shown by high-resolution transmission electron microscopy to also improve the structural integrity of the SnO2/MXene anode during cycling. The HfO2coated SnO2/MXene electrodes demonstrate a stable specific capacity of 843 mAh/g when used as Li-ion battery anodes.

Due to the low density of sulfur and the large portion of carbon-based materials used as conducting network and lithium polysulfide (LiPS) host, the practical volumetric energy density of lithium–sulfur (Li–S) batteries barely rivals the Li-ion batteries. Here, MXene (Ti3C2Tx)-based membrane with unique 3D hierarchical structure, high electronic conductivity, abundent active binding sites, fast ion transport, and high affinity for lithium polysulfides has been developed as a new host material to improve the electrochemical performance of Li-S batteries. With a density of 2.2 g cm−3, a MXene-based cathode containing 4.0 mg cm−2sulfur delivers a high volumetric capacity of 2.7 Ah cm−3after 200 cycles. Based on operandoXRD and ex-situXPS results, we find that the Ti-OH bonds present on the surface of MXene membrane can effectively trigger the LiPS transformation. Furthermore, α-S8, as the stable charge product, is first reported in MXene-based host along with its possible important role in curtailing active mass loss and enhancing cycling capability. Our results reveal that 2D MXene with rationally-designed architecture enable high volumetric capacity Li-S batteries for practical applications.

A critical limitation to conventional electrochemical double-layer capacitors is their low energy densities. This has triggered significant interest in developing new pseudocapacitive materials, which utilize faradaic mechanisms to increase their energy densities. In this work, graphitic carbon nitride (g-C3N4) and Ti3C2TxMXene are hybridized to form a unique two-dimensional (2D) heterostructure, which delivers remarkable pseudocapacitive characteristics and robust stability towards lithium storage. Interestingly, the improved kinetics is reflected by insignificant influence of (dis)charge rates on the pseudocapacitance even when testing at a 120C rate, and small peak potential offsets at high scan rates, revealing that there are no significant diffusion limitations in the heterostructure. This unexpected fast kinetics is related to the intrinsic chemical and electronic coupling effects between g-C3N4and MXene, which can synergistically improve both electron transfer and lithium diffusion kinetics compared to MXene itself.