Dual Strategies of Metal Preintercalation and In Situ Electrochemical Oxidization Operating on MXene for Enhancement of Ion/Electron Transfer and Zinc-Ion Storage Capacity in Aqueous Zinc-Ion Batteries

As an emerging two-dimensional material, MXenes exhibit enormous potentials in the fields of energy storage and conversion, due to their superior conductivity, effective surface chemistry, accordion-like layered structure, and numerous ordered nanochannels. However, interlayer accumulation and chemical sluggishness of structural elements have hampered the demonstration of the superiorities of MXenes. By metal preintercalation and in situ electrochemical oxidization strategies on V₂ CT_x , MXene has enlarged its interplanar spacing and excited the outermost vanadium atoms to achieve frequent transfer and high storage capacity of Zn ions in aqueous zinc-ion batteries (ZIBs). Benefiting from the synergistic effects of these strategies, the resulting VO_x /Mn-V₂ C electrode exhibits the high capacity of 530 mA h g^-1 at 0.1 A g^-1 , together with a remarkable energy density of 415 W h kg^-1 and a power density of 5500 W kg^-1 . Impressively, the electrode delivers excellent cycling stability with Coulombic efficiency of nearly 100% in 2000 cycles at 5 A g^-1 . The satisfactory electrochemical performances bear comparison with those in reported vanadium-based and MXene-based aqueous ZIBs. This work provides a new methodology for safe preparation of outstanding vanadium-based electrodes and extends the applications of MXenes in the energy storage field.

3D-Printed Sodiophilic V2CTx/rGO-CNT MXene Microgrid Aerogel for Stable Na Metal Anode with High Areal Capacity

Featuring a high theoretical capacity, low cost, and abundant resources, sodium metal has emerged as an ideal anode material for sodium ion batteries. However, the real feasibility of sodium metal anodes is still hampered by the uncontrolled sodium dendrite problems. Herein, an artificial three-dimensional (3D) hierarchical porous sodiophilic V₂CT_x/rGO-CNT microgrid aerogel is fabricated by a direct-ink writing 3D printing technology and further adopted as the matrix of Na metal to deliver a Na@V₂CT_x/rGO-CNT sodium metal anode. Upon cycling, the V₂CT_x/rGO-CNT electrode can yield a superior cycling life of more than 3000 h (2 mA cm^-2, 10 mAh cm^-2) with an average Coulombic efficiency of 99.54%. More attractively, it can even sustain a stable operation over 900 h at 5 mA cm^-2 with an ultrahigh areal capacity of 50 mAh cm^-2. In situ and ex situ characterizations and density functional theory simulation analyses prove that V₂CT_x with abundant sodiophilic functional groups can effectively guide the sodium metal nucleation and uniform deposition, thus enabling a dendrite-free morphology. Moreover, a full cell pairing a Na@V₂CT_x/rGO-CNT anode with a Na₃V₂(PO₄)₃@C-rGO cathode can deliver a high reversible capacity of 86.27 mAh g^-1 after 400 cycles at 100 mA g^-1. This work not only clarifies the superior Na deposition chemistry on the sodiophilic V₂CT_x/rGO-CNT microgrid aerogel electrode but also offers an approach for fabricating advanced Na metal anodes via a 3D printing method.

Surface Selenization Strategy for V2CTx MXene toward Superior Zn-Ion Storage

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.

3D V2CTx-rGO Architectures with Optimized Ion Transport Channels toward Fast Lithium-Ion Storage

Two-dimensional (2D) MXene materials show great potential in energy storage devices. However, the self-restacking of MXene nanosheets and the sluggish lithium-ion (Li⁺) kinetics greatly hinder their rate capability and cycling stability. Herein, we interlink 2D V₂CT_x MXene nanosheets with rGO to construct a 3D porous V₂CT_x-rGO composite. X-ray spectroscopy study reveals the close interfacial contact between V₂CT_x and rGO via electron transfer from V to C atoms. Benefiting from the close combination and optimized ion transport channel, V₂CT_x-rGO offers a high-rate Li⁺ storage performance and excellent cycling stability over 2000 cycles with negligible capacity attenuation. Moreover, it exhibits a dominant mechanism of intercalation pseudocapacitance and efficient Li⁺ transport proved by density functional theory calculation. This rationally designed 3D V₂CT_x-rGO has implications for the study of the MXene composite's structure and energy storage devices with high rate and stability.