Xiaohui Wang

Chinese Academy of Sciences | CAS · High-Performance Ceramic Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research

High‐rate lithium/sodium ion batteries or capacitors are the most promising functional units to achieve fast energy storage that highly depends on charge host materials. Host materials with lamellar structures are a good choice for hybrid charge storage hosts (capacitor or redox type). Emerging layered transition metal carbo‐chalcogenides (TMCC) wi...

The electrification of heavy-duty transport and aviation urgently requires new strategies to develop high-rate lithium-ion batteries (LIBs) whose performance fundamentally relies on electrode materials. However, commercially available graphite anodes still suffer from slow kinetics of lithium-ion diffusion and severe safety concerns of lithium plat...

MAX phases have emerged as promising corrosion-resistant electromagnetic interference (EMI) shielding materials. Herein, four MAX phases: Ti3SiC2, Ti3AlC2, V0.5Cr1.5AlC, and Nb4AlC3, were synthesized via solid–liquid reactions. The electrical conductivities of Ti3SiC2, Ti3AlC2, V0.5Cr1.5AlC, and Nb4AlC3 are 14.7 × 10³, 15.5 × 10³, 5.1 × 10³, 8.0 ×...

Topochemically transforming layered non-van der Waals solid into two dimensional (2D) materials involves selective etching reactions with atomic precision. The element-specific, structure-sensitive etching at nanoscale urgently requires in-depth understanding. Here, by means of density functional theory calculations and a laboratory-made operando r...

Willemite Zn2SiO4 crystallizes in such a way that Zn and Si are tetrahedrally coordinated with O in an ionic-covalent manner to form ZnO4 and SiO4 tetrahedra as the building units. The tetrahedra are corner-sharing, of which one SiO4 tetrahedron connects eight ZnO4 tetrahedra, and one ZnO4 tetrahedron links four ZnO4 tetrahedra and four SiO4 tetrah...

To explore the mechanism of phase transformation, YTa3O9 was prepared by an integrated one-step synthesis and sintering method at 1500 oC using Y2O3 and Ta2O5 powders as starting materials. High-temperature XRD patterns and Raman spectra showed that a phase transformation from orthorhombic to tetragonal took place in YTa3O9 through the bond length...

Ever increasing demands for portable electronics and electric vehicles critically require high-rate Li-ion batteries (LIBs) whose performance fundamentally depends on electrode materials. Whereas, the currently commercialized anode materials including graphite...

Novel honeycomb-structured Ag@Ti3C2Tx composites are prepared by a microwave-assisted synthesis method using polymethyl methacrylate as an expendable template. The resulting Ag@Ti3C2Tx composites retain their original spherical morphology and exhibit hydrophobic performance, low thermal conductivity, and excellent electromagnetic interference (EMI)...

MXene is a new intercalation pseudocapacitive electrode material for supercapacitor application. Intensifying fast ion diffusion is significantly essential for MXene to achieve excellent electrochemical performance. The expansion of interlayer void by traditional spontaneous species intercalation always leads to a slight increase in capacitance due...

MAX phases (Ti3SiC2, Ti3AlC2, V2AlC, Ti4AlN3, etc.) are layered ternary carbides/nitrides, which are generally processed and researched as structure ceramics. Selectively removing A layer from MAX phases, MXenes (Ti3C2, V2C, Mo2C, etc.) with two-dimensional (2D) structure can be prepared. The MXenes are electrically conductive and hydrophilic, whic...

The cobalt sulfides have become the focus materials on the lithium ion batteries application in which the Co9S8 with the nanosheet structure is the promising candidate for anode material in high-energy and high-power lithium ion batteries. By introducing a novel strategy with the organic-inorganic hybrid Co9S8-DETA (DETA= Diethylenetriamine) as the...

MXenes represent an emerging family of two-dimensional materials of transition metal carbides/carbonitrides terminated with functional groups like -O, -OH, and -F on the chemically active surface of MX slabs. As a member of the family, Nb2CTx exhibits superior lithium storage capacity over most of the other MXenes as anode materials in lithium-ion...

The advance in communication technology has triggered worldwide concern on electromagnetic wave pollution. To cope with this challenge, exploring high-performance electromagnetic (EM) wave absorbing materials with dielectric and magnetic losses coupling is urgently required. Of the EM wave absorbers, transition metal diborides (TMB2) possess excell...

LaMgAl11O19 (LMA), characterized by high melting point, low density and thermal conductivity as well as good infrared emissivity, is regarded as a potential candidate for the thermal protection of hypersonic vehicles. Nevertheless, the unsatisfied phase stability at high temperature results in declining of the emissivity below 6 μm, which limits th...

Silicon has been gaining considerable attention in the field of Li‐ion batteries. Whereas, unstable solid electrolyte interface, extremely volume change and inferior conductivity hinder its practical application. Herein, ultrasonic‐assisted fluidic colloidal coprecipitation is proposed to fabricate MXene wrapped Si nanoparticles in the manner of el...

Y2O3 is regarded as one of the potential environmental barrier coating (EBC) materials for Al2O3f/Al2O3 ceramic matrix composites owing to its high melting point and close thermal expansion coefficient to Al2O3. However, the relatively high thermal conductivity and unsatisfactory calcium–magnesium–aluminosilicate (CMAS) resistance are the main obst...

The sluggish Li‐ion diffusivity in LiFePO4, a famous cathode material, relies heavily on the employment of a broad spectrum of modifications to accelerate the slow kinetics, including size and orientation control, coating with electron‐conducting layer, aliovalent ion doping, and defect control. These strategies are generally implemented by employi...

Searching for thermal conductive materials with high electromagnetic interference (EMI) shielding effectiveness (SE) is the key to protect electronic equipment against electromagnetic pollution and excess heat emission. Herein, NdB6/SiO2 bulks with high EMI SE and thermal conductivity which also exhibit good mechanical properties were prepared by l...

REB2C2 (RE = Y and lanthanides) compounds have gained attention for their unique layered crystal structure. However, there have been few reports about Raman spectroscopy of REB2C2 compounds up to now. Here, the Raman spectrum of YbB2C2 is obtained by a micro-Raman spectroscope and the first-principles calculations. Raman active vibrational modes of...

The critical requirements for the environmental barrier coating (EBC) materials of silicon-based ceramic matrix composites (CMCs) include good tolerance to harsh environments, thermal expansion matches with the interlayer mullite, good high-temperature phase stability, and low thermal conductivity. Cuspidine-structured rare-earth aluminates RE4Al2O...

The use of precious metals in many areas such as printed circuit boards, catalysts and targeted drugs, is increasing due to their unique physical and chemical properties, but their recovery remains a great challenge. Here we report a sandwiched Ti3C2Tx MXene/carbon nanotube (CNT) hybrid membrane, where the CNT isolates and supports the MXene sheets...

MXenes refer to a family of 2D transition metal carbides/nitrides that are rich in chemistry. The first member of the family, Ti3C2Tx, was reported in 2011. Since then MXenes have opened up an exciting new field in 2D inorganic functional materials by virtue of their intrinsic electronic conductivity, superior hydrophilicity, rich surface chemistry...

Rare-earth tantalates and niobates (RE3TaO7 and RE3NbO7) have been considered as promising candidate thermal barrier coating (TBC) materials in next generation gas-turbine engines due to their ultra-low thermal conductivity and better thermal stability than yttria-stabilized zirconia (YSZ). However, the low Vickers hardness and toughness are the ma...

Al5BO9 is a promising thermal sealing material for hypersonic vehicles due to its low density, theoretically predicted low shear modulus, and low thermal conductivity. However, experimental investigations on the mechanical and thermal properties of bulk Al5BO9 have not been carried out. Herein, we report the mechanical and thermal properties of bul...

The critical requirements for the environmental barrier coating (EBC) materials of silicon-based ceramic matrix composites (CMCs) including good tolerance to harsh environments, thermal expansion match with the interlayer mullite, good high-temperature phase stability and low thermal conductivity. Cuspidine-structured rare-earth aluminates RE 4 Al...

The critical requirements for the environmental barrier coating (EBC) materials of silicon-based ceramic matrix composites (CMCs) including good tolerance to harsh environments, thermal expansion match with the interlayer mullite, good high-temperature phase stability and low thermal conductivity. Cuspidine-structured rare-earth aluminates RE 4 Al...

Developing nano‐ or atom‐scale Pt‐based electrocatalysts for hydrogen evolution reaction (HER) is of considerable importance to mitigate the issues associated with low abundance of Pt. Here, a protocol for constructing a hierarchical Pt––MXene–single‐walled carbon nanotubes' (SWCNTs) heterostructure for HER catalysts is presented. In the heterostru...

Two-dimensional (2D) transition-metal carbide materials termed MXene have attracted huge attention in the field of electrochemical energy storage. To this end, MXenes whose capability depends on the M transition elements represent a new paradigm extending beyond the realm of oft-explored elemental 2D materials beginning with graphene. However, the...

Yttrium aluminum perovskite (YAlO3) is a promising candidate material for environmental barrier coatings (EBCs) to protect Al2O3f/Al2O3 ceramic matrix composites (CMCs) from the corrosion of high-temperature water vapor in combustion environments. Nevertheless, the relative high thermal conductivity is a notable drawback of YAlO3 for environmental...

Electrochemical pseudocapacitors store energy via intercalation or electrosorption and Faradaic charge transfer with redox reactions. MXenes represent the promising intercalation pseudocapacitive electrode materials for supercapacitors due to their ultrahigh theoretical capacitances. Achieving the high capacitance will greatly advance the large-sca...

Designing efficient electrocatalysts with low Pt loadings for hydrogen evolution reaction (HER) is urgently required for renewable and sustainable energy conversion. Here, we report a strategy that Pt nanoparticulates are spontaneously immobilized on porous MXene/MAX monolith as HER catalysts by utilizing the redox reaction between Ti3C2Tx MXene an...

Low thermal conductivity, matched thermal expansion coefficient and good compatibility are general requirements for the coating materials and interphase materials for Al2O3/Al2O3 ceramic matrix composites. In this work, a novel high-entropy rare-earth phosphate monazite ceramic (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4 is designed and successfully synthesized...

MXenes hold immense promise for diverse applications that highly depend on the compositional and structural features of both the carbide frame and interlayer functional groups. Here, we show that electron probe can controllably impart metal atoms in the carbide slabs to the functional group slabs in MXene multilayers. The multilayers consist of man...

MXenes have emerged as promising high-volumetric-capacitance supercapacitor electrode material, whereas their voltage windows are not wide. This disadvantage prevents MXenes from being made into aqueous symmetric supercapacitors with high energy density. To attain high energy density, constructing asymmetric supercapacitors is a reliable design cho...

Phase pure and well crystalline Cr2AlB2 powders are synthesized by heating the mixtures of CrB and Al powders at 900 °C. Cr2AlB2 exhibits nanolaminated morphology which transforms from flake-like crystallite to needle-like grain with the increase of holding time. The morphology-structure relationships of Cr2AlB2 are delicately discussed. Meanwhile,...

MXenes, as an emerging family of conductive two-dimensional materials, hold promise for late-model electrode materials in Li-ion batteries. A primary challenge hindering the development of MXenes as electrode materials is that a complete understanding of the intrinsic storage mechanism underlying the charge/discharge behavior remains elusive. This...

Hydrothermal synthesis of high-performance LiFePO4 nanocrystals in pure water represents a green and sustainable approach to the model cathode material for lithium ion batteries. However, fundamental and technological hurdles in terms of yield and cost remain to be overcome, though strenuous efforts have been devoted to the challenging approach. He...

Two-dimensional materials (such as graphene, MXenes) usually perform remarkably well as catalysts compared with their three-dimensional counterparts, due to their unique physical and chemical properties. In this work, Ti3AlC2-derived Ti3C2Tx MXene (T=O, OH, F) was prepared and employed in the direct dehydrogenation of ethylbenzene. A 92 μmol m-2 h-...

Submicron Ti 3 AlC 2 grains are grown in molten salt. Etching the grains gives rise to small-sized Ti 3 C 2Tx MXene particulates with capacitance more than twice the large ones derived from conventional high-temperature...

MXenes, an emerging class of conductive two-dimensional materials, have been regarded as promising candidates in the field of electrochemical energy storage. The electrochemical performance of their representative, Ti3C2Tx, where T stands for the surface termination group of F, O, or OH, strongly relies on termination mediated surface functionaliza...

[100]Pnma is believed to be a tough diffusion direction for Li⁺ in LiFePO4, leading to the belief that the rate performance of [100]-oriented LiFePO4 is poor. Recent work revealed that reducing the dimension of the LiFePO4 phase to 12 nm in the [100] direction increased the extent of Li solid solution between LiFePO4 and FePO4, which produced an in...

Flexible fiber-shaped supercapacitors are promising energy-storage devices in wearable smart electronics. Exploring the knittable fiber-type electrode with high capacitance would promote the wide-scale application of such capacitors. Herein, a solution-processed methodology to fabricate all-solid-state, flexible, and fiber-based supercapacitors emp...

Nb-based ‘312’ MAX phase has not been recognized so far, raising a hypothesis that Nb doping would destabilize the isostructural Ti3AlC2. Here we report that (Ti1−xNbx)3AlC2 could persist with a doping limitation up to x = 0.15. As demonstrated by HAADF-STEM analysis, Nb dopants homogeneously distribute among polycrystalline grains at the microscal...

Achieving controlled synthesis of nanocrystals requires insight into the formation mechanism. The formation mechanism that determines the nucleation and growth for LiFePO4 nanoflakes has been visualized by using Li3PO4 microprisms as the precursor and as a solid ‘tracer’. The sluggishly dissolved precursor facilitates simultaneous heterogeneous nuc...

Directly controlling the morphology and orientation of nanocrystals is a key step for their tailored applications. Achieving such controlled synthesis requires an insight into the formation mechanism. Using Li3PO4 microprisms as solid precursor sealed in capillary tube, we definitely unravel structural and morphological evolution from the precursor...

Layered compounds play pivotal roles as precursors for producing 2D materials through mechanical exfoliation (micro-mechanical cleavage) or chemical approaches. Therefore, searching for layered compounds with sharp anisotropic chemical bonding and properties becomes emergent. In this work, the stability, electronic structure, elastic properties, an...

Porous LiMn2O4 hollow microspheres were facilely prepared by incorporation of Li and Mn elements into a spherical polymeric precursor through copolymerization of lithium and manganese acetates with resorcinol and hexamethylenetetramine and then burning off the organic matrix at appropriate temperatures in air. The LiMn2O4 inherited the spherical mo...

Carbon-vacancy-bearing Nb4AlC3−x has the best high-temperature mechanical robustness among MAX phases. The existing form of the vacancies has been long overlooked. Recently, the vacancies in Nb4AlC3−x have been identified to be ordered, establishing an ordered compound Nb12Al3C8. Here, the spatial distribution of the ordered vacancies and their inf...

Polycrystalline YAl3C3 ceramic was fabricated by an in situ hot-pressing process using YH2, Al, and C powders as starting materials. The crystal structure of YAl3C3 was determined by X-ray diffraction and ab initio calculations to be a hexagonal space group P63/mmc. The lattice parameters are a = b = 3.424 Å, c = 17.291 Å, and the atomic positions...

Success in the exfoliation of the stacked T-functionalized titanium carbide MXenes Tin+1CnT2 (T = OH, O, and F) would potentially extend their application scope, which requires an understanding of the nature of interlayer coupling. Here, we report for the first time the intrinsic interlayer coupling in pristine MXenes on the basis of first-principl...

Tilt-dominated grain boundaries have been investigated in depth in the deformation of MAX phases. In stark contrast, another important type of grain boundaries, twist grain boundaries, have long been overlooked. Here, we report on the observation of small angle twist sub-grain boundaries in a typical MAX phase Ti3AlC2 compressed at 1200 °C, which c...

[100] is believed to be a tough diffusion direction for Li(+) in LiFePO4, leading to the belief that the rate performance of [100]-oriented LiFePO4 is poor. Here we report the fabrication of 12-nm-thick [100]-oriented LiFePO4 nanoflakes by a simple one-pot solvothermal method. The nanoflakes exhibit unexpectedly excellent electrochemical performanc...

Structure and vibrational dynamics of T-terminated titanium carbide monosheets Ti2CT2 (T = O, F, OH) are studied by means of first-principles calculations to understand their inherent relation. Terminations modulate the crystal structures through the redistribution of valence electron density among the atoms in the monosheets, particularly Ti atoms...

Stacked two-dimensional titanium carbide is an emerging conductive material for electrochemical energy storage which requires an understanding of the intrinsic electronic conduction. Here we report the electronic conduction properties of stacked Ti3C2T2 (T = OH, O, F) with two distinct stacking sequences (Bernal and simple hexagonal). On the basis...

The conventional wisdom to tailor the properties of binary transition metal carbides by order-disorder phase transformation has been inapplicable for the machinable ternary carbides (MTCs) due to the absence of ordered phase in bulk sample. Here, the presence of an ordered phase with structural carbon vacancies in Nb4AlC3-x (x ≈ 0.3) ternary carbid...

Binder-free MXene films comprising Ti3C2Tx nanoflakes in a face-to-face fashion, achieved by a simple dropping-mild baking approach, exhibit high gravimetric capacitances up to 499 F/g with excellent cyclability and rate performances. The entire electrodes including Ni-foam and MXene film show volumetric capacitances in the range of 84-226 F/cm3 de...

We report on strain-rate-dependent compression deformation behavior of Ti3AlC2 at 1000°C–1200°C. At 1000°C and high strain rate (10−2 or 10−3 s−1), Ti3AlC2 deforms in a nonplastic manner. Upon increasing temperature and reducing strain rate, Ti3AlC2 exhibits a limited plasticity. For instance, the true plastic strain at 1200°C and 10−4 s−1 is only...

A novel method for preparing high porosity and high strength porous YB4 ceramics was developed combining the in-situ borothermal reaction synthesis and high temperature partial sintering. Since pores could be produced by gasses such as B2O3 and CO generated in the reaction between Y2O3 and B4C, high porosity porous YB4 ceramics were obtained after...

We present a comparative study on the static and dynamical properties of bare Ti3C2 and T-terminated Ti3C2T2 (T = O, F, OH) monosheets using density functional theory calculations. First, the crystal structures are optimized to be of trigonal configurations (P3[combining macron]m1), which are thermodynamically and dynamically stable. It is demonstr...

A cost-efficient strategy using elemental powders of Ti, Al, and graphite as starting materials is proposed to fabricate conductive Ti2AlC honeycomb monolith. The honeycomb structure is extruded from a plastic feed material composed of the initial powders and several additives. Phase-pure Ti2AlC can be obtained with a direct heating schedule, but t...

Due to its high twin boundary energy, the absence of Σ3{112} in covalently bonded TiC has been theoretically adjudged. Here, we report the observation of TiC twin boundaries that consist of Σ3{112} and Σ3{111} steps in a faceted and inclined manner. The twin interface in the transition region is not flat, wherein periodic contrasts resulting from o...

We design four-group experiments to understand the morphological and orientational diversity of LiFePO4 crystallites in hydrothermal and/or solvothermal syntheses. In the solvothermal synthesis in which water is highly deficient, the starting Li3PO4 nanoparticle likely directly evolves into LiFePO4 through an in situ transition mechanism. In contra...

Wear performance of a near equi-volume TiC–Ni2AlTi cermet with minor NiAl was evaluated by reciprocal sliding against Si3N4 balls. Both coefficient of friction (COF) and specific wear rate (SWR) decrease with the applied load in the range from 5 to 20 N, reaching minimums of 0.34 and 2.2 × 10−6 mm3/Nm, respectively, at 20 N. To understand the novel...

Titanium carbide (TiC) twins are believed to be extremely unstable because of their high twin boundary energy. Here, we report that TiC twins are always presented in platelets with dimensions of 2–3 μm in length and less than 300 nm in width. In-depth microstructural characterizations by high-resolution transmission electron microscopy demonstrate...

Nanolaminated Ti3AlC2 honeycomb monolith with parallel and uniform holes has been prepared through a facile extrusion route by using Ti3AlC2 powder as the main raw material. The fabricated honeycomb monolith has high compressive strength of 133±11 and 59±9 MPa, along and perpendicular to the extrusion direction, respectively. It also has good elect...

Micro-Raman spectroscopic study and lattice dynamics calculations were conducted to study a recently identified layered ternary carbide, Ti5Al2C3. The experimental Raman shifts were remarkably consistent with the calculated values. Polarized Raman spectrum was collected in the polycrystalline sample, which confirmed the theoretical symmetry assignm...

Crystal structure of Ti5Al2C3 was determined by means of X-ray powder diffraction (XRPD), transmission electron microscopy (TEM) and ab initio calculations. In contrast to the already known P63/mmc space group that the MAX phases crystallize, it was demonstrated that the R3¯m space group could better satisfy the experimental data. The lattice param...

The thermal and mechanical properties of β-Yb2Si2O7 were investigated using a combination of first-principles calculations and experimental investigations. Theoretically, anisotropic chemical bonding and elastic properties, weak interatomic (010) and (001) planes in the crystal structure, damage tolerance, and low thermal conductivity are predicted...

Superconductivity of Nb2AlC has been previously reported, but the origin is not clear. In this paper, in situ Raman spectra of Nb2AlC are measured in the temperature range from 80 to 380 K at ambient pressure. The line-width of E2g (ω1) mode increases with temperature which originates from the anharmonic phonon–phonon scattering. On the contrary th...

Reactivity and stability of selected nanolaminated carbides/nitrides (MAX phases) in a concentrated HCl solution are investigated under hydrothermal conditions. The reactivity of the MAX phases strongly depends on the reactivity of A element (Al or Si) with the acid. Al-containing MAX phases suffered severe corrosion, whereas Si-containing Ti3SiC2...

Ti3AlC2 has low density, high elastic modulus, quasi‐ductility, and excellent oxidation resistance. However, its relatively low strength needs to be improved. In this study, dense Ti3AlC2–Y4Al2O9 composites (containing 2.5, 5, and 7.5 wt% Y4Al2O9) were synthesized by hot pressing/solid‐liquid reaction using Ti,Al,C, and Y2O3 as initial materials. W...

Titanate nanowires prepared by hydrothermal treatment of TiO2 in NaOH solution have attracted intensive attentions, but the formation mechanism is still under debate. Herein, we report an in-depth study on the formation mechanism through investigating the hydrothermal behavior of various Ti-containing precursors in basic solutions, including Ti, Ti...

Insights into high temperature oxidation of Al2O3-forming Ti3AlC2 are made to understand the sub-parabolic oxidation rate and the absence of Al-depletion zone beneath the protective Al2O3 scale. The scale results from selective oxidation of Al that migrated from Ti3AlC2 according to Ti3AlC2 + 3/2y O2 → Ti3Al1−yC2 + 1/2y Al2O3, leaving Al vacancy in...

A new compound, Ti5Al2C3, is identified in the bulk sample prepared by reactive hot pressing a mixture of Ti,Al and graphite powders with a nominal molar ratio of Ti:Al:C = 5:2.15:2.78 at 1580°C for 1 h. This compound follows the crystal structure of other MAX phases. The lattice parameters are a = 0.3038 nm, c = 3.2261 nm, and the atomic positions...

Improving electrochemical properties of hydrothermally synthesized LiFePO(4) powders is of immense technological significance and has been a subject of much scientific inquiry for many years. As reported previously, reversing the feeding sequence of starting materials and/or introducing ethylene glycol (EG) could significantly improve the electroch...

We describe the synthesis of hexagonal NaNbO3 by mixing reactants at high temperature under hydrothermal conditions. The morphological and structural evolution of crystalline NaNbO3 was studied in detail using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, high-resolution trans...

In situ Raman spectra of Al3BC3 have been measured at elevated temperatures of up to 1000 °C and high pressures of up to 32 GPa. It is shown that there are no abnormal phonon shifts when the measurement conditions were up to 1000 °C or up to 27 GPa, which indicates a good structural stability of Al3BC3 at high temperatures and pressures. The Raman...

Due to the large volume fraction of electrochemically inactive materials involved in conventional thin calendered electrodes, it is challenging to approach high volumetric capacity. Here, we report a proof-of-concept study on LiFePO4-based bulk electrodes, as opposed to conventional thin calendered electrodes. The bulk electrodes are hierarchically...

The low-temperature hydrothermal synthesis method has been drawing ever-growing attention due to the fact that it has many advantages over conventional methods for preparing promising cathode material LiFePO4. However, the mechanism for hydrothermal synthesis of LiFePO4 remains unclear. Here, the hydrothermal reaction mechanism of LiFePO4 is system...