三元层状材料结构调控及性能研究进展

MAX/MAB相是一类非范德华三元层状材料,具有丰富的元素组成和晶体结构,兼具陶瓷和金属的物理性质,在高温、强腐蚀、辐照等极端环境中极具应用潜力。近年来,由MAX/MAB相衍生的二维(2D)材料(MXene和MBene)在材料物理与材料化学领域引起了广泛兴趣,已经成为继石墨烯和过渡金属硫族化合物之后最受关注的二维范德华材料。MAX/MAB相材料结构调控不仅对这类非范德华层状材料本征性能产生重要影响,而且对其衍生的二维范德华材料结构功能特性研究也具有重要价值。本文归纳和总结了MAX/MAB相层状材料在结构调控、理论计算和应用基础研究等方向的最新科研进展,并展望了该类层状材料未来发展方向。

Pseudocapacitive Heteroatom-Doped Carbon Cathode for Aluminum-Ion Batteries with Ultrahigh Reversible Stability

Aluminum(Al)-ion batteries have emerged as a potential alternative to conventional ion batteries that rely on less abundant and costly materials like lithium.Nonetheless,given the nascent stage of advancement in Al-ion batteries(AIBs),attaining electrode materials that can leverage both intercalation capacity and structural stability remains challenging.Herein,we demonstrate a C3N4-derived layered N,S heteroatom-doped carbon,obtained at different pyrolysis temperatures,as a cathode material for AIBs,encompassing the diffusion-controlled intercalation and surface-induced capacity with ultrahigh reversibility.The developed layered N,S-doped corbon(N,S-C)cathode,synthesized at 900℃,delivers a specific capacity of 330 mAhg^(-1)with a relatively high coulombic efficiency of~85%after 500 cycles under a current density of 0.5 A g^(-1).Owing to its reinforced adsorption capability and enlarged interlayer spacing by doping N and S heteroatoms,the N,S-C900 cathode demonstrates outstanding energy storage capacity with excellent rate performance(61 mAhg^(-1)at 20 A g^(-1))and ultrahigh reversibility(90 mAhg^(-1)at 5Ag^(-1)after 10000cycles).

Thick Electrodes of a Self-Assembled MXene Hydrogel Composite for High-Rate Energy Storage

Supercapacitors based on two-dimensional MXene(Ti_(3)C_(2)T_(z))have shown extraordinary performance in ultrathin electrodes with low mass loading,but usually there is a significant reduction in high-rate performance as the thickness increases,caused by increasing ion diffusion limitation.Further limitations include restacking of the nanosheets,which makes it challenging to realize the full potential of these electrode materials.Herein,we demonstrate the design of a vertically aligned MXene hydrogel composite,achieved by thermal-assisted self-assembled gelation,for high-rate energy storage.The highly interconnected MXene network in the hydrogel architecture provides very good electron transport properties,and its vertical ion channel structure facilitates rapid ion transport.The resulting hydrogel electrode show excellent performance in both aqueous and organic electrolytes with respect to high capacitance,stability,and high-rate capability for up to 300μm thick electrodes,which represents a significant step toward practical applications.

MXene-based symmetric supercapacitors with high voltage and high energy density

MXene-based aqueous symmetric supercapacitors(SSCs)are attractive due to their good rate performances and green nature.However,it remains a challenge to reach voltages much over 1.2 V,which significantly diminishes their energy density.Herein,we report on Mo_(1.33)CTz MXene-based SSCs possessing high voltages in a 19.5 M LiCl electrolyte.Benefiting from the vacancy-rich structure and high stable potential window of Mo_(1.33)CTz,the obtained SSCs deliver a maximum energy density of>38.2 mWh cm^(-3) at a power density of 196.6 mW cm^(-3) under an operating voltage of 1.4 V,along with excellent rate performance and impressive cycling stability.This highly concentrated LiCl electrolyte is also applicable to Ti_(3)C_(2)Tz,the most widely studied MXene,achieving a maximum energy density of>41.3 mWh cm^(-3) at a power density of 165.2 mW cm^(-3) with an operating voltage of 1.8 V.The drop in energy density with increasing power in the Ti_(3)C_(2)Tz cells was steeper than for the Mo-based cells.This work provides a roadmap to develop superior SSCs with high voltages and high energy densities.

Drawing highly ordered MXene fibers from dynamically aggregated hydrogels

Assembly of two-dimensional (2D) nanomaterials into well-organized architectures is pivotal for controlling their function and enhancing performance. As a promising class of 2D nanomaterials, MXenes have attracted significant interest for use in wearable electronics due to their unique electrical and mechanical properties. However, facile approaches for fabricating MXenes into macroscopic fibers with controllable structures are limited. In this study, we present a strategy for easily spinning MXene fibers by incorporating polyanions. The introduction of poly(acrylic acid) (PAA) into MXene colloids has been found to alter MXene aggregation behavior, resulting in a reduced concentration threshold for lyotropic liquid crystal phase. This modification also enhances the viscosity and shear sensitivity of MXene colloids. Consequently, we were able to draw continuous fibers directly from the gel of MXene aggregated with PAA. These fibers exhibit homogeneous diameter and high alignment of MXene nanosheets, attributed to the shear-induced long-range order of the liquid crystal phase. Furthermore, we demonstrate proof-of-concept applications of the ordered MXene fibers, including textile-based supercapacitor, sensor and electrical thermal management, highlighting their great potential applied in wearable electronics. This work provides a guideline for processing 2D materials into controllable hierarchical structures by regulating aggregation behavior through the addition of ionic polymers.

Colorless-to-colorful switching of electrochromic MXene by reversible ion insertion

Electrochromic(EC)materials that change color with voltage have been widely studied for use in dynamic windows.However,colorless-to-colorful switching with high contrast ratio is generically unattainable,especially for colorless-to-black electrochromic materials with an ultrahigh contrast ratio over the entire visible region.In this work,we developed Nb_(1.33)C MXene-based dynamic windows with colorless-to-black switching of up to 75%reversible change in transmittance from 300 to 1,500 nm.By exploring the electrochromic effects of different electrolytes through in situ optical changes and electrochemical quartz crystal microbalance(EQCM),it is found that electrochromic behavior is greatly influenced by the extent of reversible Li^(+)insertion/deinsertion between the two-dimensional Nb_(1.33)C MXene nanosheets.In addition,a colorless-to-black EC device based on Nb_(1.33)C with an overall integrated contrast ratio over 80%was successfully constructed by a solution-processable spin coating method.This work enables a simple route to fabricate MXene-based high-performance electrochromic smart windows,which is important for further expanding the application of MXenes to optoelectronic and photonic applications.

Finding stable multi-component materials by combining cluster expansion and crystal structure predictions

A desired prerequisite when performing a quantum mechanical calculation is to have an initial idea of the atomic positions within an approximate crystal structure.The atomic positions combined should result in a system located in,or close to,an energy minimum.However,designing low-energy structures may be a challenging task when prior knowledge is scarce,specifically for large multi-component systems where the degrees of freedom are close to infinite.In this paper,we propose a method for identification of low-energy crystal structures within multi-component systems by combining cluster expansion and crystal structure predictions with density-functional theory calculations.Crystal structure prediction searches are applied to the Mo_(2)AlB_(2) and Sc2AlB_(2) ternary systems to identify candidate structures,which are subsequently used to explore the quaternary(pseudo-binary)(Mo_(x)Sc_(1–x))2AlB_(2) system through the cluster expansion formalism utilizing the ground-state search approach.Furthermore,we show that utilizing low-energy structures found within the cluster expansion ground-state search as seed structures within crystal structure predictions of(Mo_(x)Sc_(1–x))2AlB_(2) can significantly reduce the computational demands.With this combined approach,we not only correctly identified the recently discovered Mo_(4/3)Sc_(2/3)AlB_(2) i-MAB phase,comprised of in-plane chemical ordering of Mo and Sc and with Al in a Kagomélattice,but also predict additional low-energy structures at various concentrations.This result demonstrates that combining crystal structure prediction with cluster expansion provides a path for identifying low-energy crystal structures in multi-component systems by employing the strengths from both frameworks.

Role of chloride on the instability of blue emitting mixed‑halide perovskites

Although perovskite light-emitting diodes(PeLEDs)have seen unprecedented development in device efciency over the past decade,they sufer signifcantly from poor operational stability.This is especially true for blue PeLEDs,whose operational lifetime remains orders of magnitude behind their green and red counterparts.Here,we systematically investigate this efciency-stability discrepancy in a series of green-to blue-emitting PeLEDs based on mixed Br/Cl-perovskites.We fnd that chloride incorporation,while having only a limited impact on efciency,detrimentally afects device stability even in small amounts.Device lifetime drops exponentially with increasing Cl-content,accompanied by an increased rate of change in electrical properties during operation.We ascribe this phenomenon to an increased mobility of halogen ions in the mixed-halide lattice due to an increased chemically and structurally disordered landscape with reduced migration barriers.Our results indicate that the stability enhancement for PeLEDs might require diferent strategies from those used for improving efciency.