Nitrogen doping has been proven to be a facile modification strategy to improve the electrochemical performance of 2D MXenes, a group of promising candidates for energy storage applications. However, the underlying mechanisms, especially the positions of nitrogen dopants, and its effect on the electrical properties of MXenes, are still largely unexplored. Herein, a comprehensive study is carried out to disclose the nitrogen doping mechanism in Ti 3 C 2 MXene, by employing theoretical simulation and experimental characterization. Three possible sites are found in Ti 3 C 2 T x (T = F, OH, and O) to accommodate the nitrogen dopants: lattice substitution (for carbon), function substitution (for-OH), and surface absorption (on-O). Moreover, electrochemical test results confirm that all the three kinds of nitrogen dopants are favorable for improving the specific capacitance of the Ti 3 C 2 electrode, and the underlying factors are successfully distinguished. By revealing the nitrogen doping mechanisms in Ti 3 C 2 MXene, this work provides theoretical guidelines for modulating the electrochemical properties of MXene materials for energy storage applications.

The realizing of high‐performance rechargeable aqueous zinc‐ion batteries (ZIBs) with high energy density and long cycling life is promising but still challenging due to the lack of suitable layered cathode materials. The work reports the excellent zinc‐ion storage performance as‐observed in few‐layered ultrathin VSe2 nanosheets with a two‐step Zn2+ intercalation/de‐intercalation mechanism verified by ex situ X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS) characterizations. The VSe2 nanosheets exhibit a discharge plateau at 1.0–0.7 V, a specific capacity of 131.8 mAh g−1 (at 0.1 A g−1), and a high energy density of 107.3 Wh kg−1 (at a power density of 81.2 W kg−1). More importantly, outstanding cycle stability (capacity retention of 80.8% after 500 cycles) without any activation process is achieved. Such a prominent cyclic stability should be attributed to its fast Zn2+ diffusion kinetics (DZn2+ ≈ 10−8 cm−2 s−1) and robust structural/crystalline stability. Density functional theory (DFT) calculation further reveals a strong metallic characteristic and optimal zinc‐ion diffusion pathway with a hopping energy barrier of 0.91 eV. The present finding implies that 2D ultrathin VSe2 is a very promising cathode material in ZIBs with remarkable battery performance superior to other layered transitional metal dichalcogenides.

2D materials with atomically precise thickness and tunable chemical composition hold promise for potential applications in nanoenergy. Herein, a bilayer-structured VOPO 4 ⋅2H 2 O (bilayer-VOP) nanosheet is developed with high-concentration oxygen vacancies ([Vo˙˙]) via a facile liquid-exfoliation strategy. Galvanostatic intermittent titration technique study indicates a 6 orders of magnitude higher zinc-ion coefficient in bilayer-VOP nanosheets (4.6 × 10 −7 cm −2 s −1 ) compared to the bulk counterpart. Assistant density functional theory (DFT) simulation indicates a remarkably enhanced electron conductivity with a reduced bandgap of ≈0.2 eV (bulk sample: 1.5 eV) along with an ultralow diffusion barrier of ≈0.08 eV (bulk sample: 0.13 eV) in bilayer-VOP nanosheets, thus leading to superior diffusion kinetics and electrochemical performance. Mott-Schottky (impedance potential) measurement also demonstrates a great increase in electronic conductivity with ≈57-fold increased carrier concentration owing to its high concentration [Vo˙˙]. Benefited by these unique features, the rechargeable zinc-ion battery composed of bilayer-VOP nanosheets cathode exhibits a remarkable capacity of 313.6 mAh g −1 (0.1 A g −1 ), an energy density of 301.4 Wh kg −1 , and a prominent rate capability (168.7 mAh g −1 at 10 A g −1 ).

Layered vanadium phosphate (VOPO4•2H2O) is reported as a promising cathode material for rechargeable aqueous Zn2+ batteries (ZIBs) owing to its unique layered framework and high discharge plateau. However, its sluggish...

Oxygen evolution electrocatalysts are central to overall water splitting, and they should meet the requirements of low cost, high activity, high conductivity, and stable performance. Herein, a general, selenic‐acid‐assisted etching strategy is designed from a metal–organic framework as a precursor to realize carbon‐coated 3d metal selenides MmSen (Co0.85Se1−x, NiSe2−x, FeSe2−x) with rich Se vacancies as high‐performance precious metal‐free oxygen evolution reaction (OER) electrocatalysts. Specifically, the as‐prepared Co0.85Se1−x@C nanocages deliver an overpotential of only 231 mV at a current density of 10 mA cm−2 for the OER and the corresponding full water‐splitting electrolyzer requires only a cell voltage of 1.49 V at 10 mA cm–2 in alkaline media. Density functional theory calculation reveals the important role of abundant Se vacancies for improving the catalytic activity through improving the conductivity and reducing reaction barriers for the formation of intermediates. Although phase change after long‐term operation is observed with the formation of metal hydroxides, catalytic activity is not obviously affected, which strengthens the important role of the carbon network in the operating stability. This study provides a new opportunity to realize high‐performance OER electrocatalysts by a general strategy on selenic acid etching assisted vacancy engineering.

for electron and charge transfer. Therefore, the OV-T n QDs@ PCN/S cathode delivers a superb long-term cycling stability (88% capacity retention over 1000 cycles at 2C) under a S-mass loading of 2.2 mg cm −1 and an E/S ratio of 10 µL mg −1 . In addition, the cathode exhibits good Li + storage at high S-mass loading (4.8 mg cm −1 ) and lean electrolyte (E/S ratio: 4.5 µL mg −1 ), demonstrating its great potential for practical implementation. Our strategy may be extended to other MXenes (e.g., Ti 3 CNT x , Nb 2 CT x , and V 2 CT x ) and pave the way to realize the facile synthesis of QDs with rich OVs for advanced Li-S batteries.

MXenes are promising cathode materials for aqueous zinc-ion batteries (AZIBs) owing to their layered structure, metallic conductivity, and hydrophilicity. However, they suffer from low capacities unless they are subjected to electrochemically induced second phase formation, which is tedious, time-consuming, and uncontrollable. Here we propose a facile one-step surface selenization strategy for realizing advanced MXene-based nanohybrids. Through the selenization process, the surface metal atoms of MXenes are converted to transition metal selenides (TMSes) exhibiting high capacity and excellent structural stability, whereas the inner layers of MXenes are purposely retained. This strategy is applicable to various MXenes, as demonstrated by the successful construction of VSe2@V2CT x , TiSe2@Ti3C2T x , and NbSe2@Nb2CT x . Typically, VSe2@V2CT x delivers high-rate capability (132.7 mA h g–1 at 2.0 A g–1), long-term cyclability (93.1% capacity retention after 600 cycles at 2.0 A g–1), and high capacitive contribution (85.7% at 2.0 mV s–1). Detailed experimental and simulation results reveal that the superior Zn-ion storage is attributed to the engaging integration of V2CT x and VSe2, which not only significantly improves the Zn-ion diffusion coefficient from 4.3 × 10–15 to 3.7 × 10–13 cm2 s–1 but also provides sufficient structural stability for long-term cycling. This study offers a facile approach for the development of high-performance MXene-based materials for advanced aqueous metal-ion batteries.

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