2D MXene-containing polymer electrolytes for all-solid-state lithium metal batteries.

Nanocomposite polymer electrolytes (CPEs) are promising materials for all-solid-state lithium metal batteries (LMBs) due to their enhanced ionic conductivities and stability to the lithium anode. MXenes are a new two-dimensional, 2D, family of early transition metal carbides and nitrides, which have a high aspect ratio and a hydrophilic surface. Herein, using a green, facile aqueous solution blending method, we uniformly dispersed small amounts of Ti3C2T x into a poly(ethylene oxide)/LiTFSI complex (PEO20-LiTFSI) to fabricate MXene-based CPEs (MCPEs). The addition of the 2D flakes to PEO simultaneously retards PEO crystallization and enhances its segmental motion. Compared to the 0D and 1D nanofillers, MXenes show higher efficiency in ionic conductivity enhancement and improvement in the performance of LMBs. The CPE with 3.6 wt% MXene shows the highest ionic conductivity at room temperature (2.2 × 10-5 S m-1 at 28 °C). An LMB using MCPE with only 1.5 wt% MXene shows rate capability and stability comparable with that of the state-of-the-art CPELMBs. We attribute the excellent performance to the 2D geometry of the filler, the good dispersion of the flakes in the polymer matrix, and the functional group-rich surface.

Tailoring Structure, Composition, and Energy Storage Properties of MXenes from Selective Etching of In-Plane, Chemically Ordered MAX Phases.

The exploration of 2D solids is one of our time's generators of materials discoveries. A recent addition to the 2D world is MXenes that possses a rich chemistry due to the large parent family of MAX phases. Recently, a new type of atomic laminated phases (coined i-MAX) is reported, in which two different transition metal atoms are ordered in the basal planes. Herein, these i-MAX phases are used in a new route for tailoriong the MXene structure and composition. By employing different etching protocols to the parent i-MAX phase (Mo2/3 Y1/3 )2 AlC, the resulting MXene can be either: i) (Mo2/3 Y1/3 )2 C with in-plane elemental order through selective removal of Al atoms or ii) Mo1.33 C with ordered vacancies through selective removal of both Al and Y atoms. When (Mo2/3 Y1/3 )2 C (ideal stoichiometry) is used as an electrode in a supercapacitor-with KOH electrolyte-a volumetric capacitance exceeding 1500 F cm-3 is obtained, which is 40% higher than that of its Mo1.33 C counterpart. With H2 SO4 , the trend is reversed, with the latter exhibiting the higher capacitance (≈1200 F cm-3 ). This additional ability for structural tailoring will indubitably prove to be a powerful tool in property-tailoring of 2D materials, as exemplified here for supercapacitors.

Alkali-induced crumpling of Ti3C2Tx (MXene) to form 3D porous networks for sodium ion storage.

We present a novel strategy for constructing three-dimensional (3D) porous Ti3C2Tx (MXene) networks by alkali-induced crumpling of Ti3C2Tx nanosheets. The 3D porous Ti3C2Tx networks display high capacity and outstanding rate performance as anode materials for sodium-ion batteries.

Pressure-induced shear and interlayer expansion in Ti3C2 MXene in the presence of water.

Pseudo-negative compressibility in layered materials is a phenomenon typically limited to in situ high-pressure experiments in some clay minerals and carbon-based materials. We show that the MXene Ti3C2T x expands along its crystallographic c direction when compressed in the presence of H2O. This expansive effect occurs when a mixture of powders and excess water is quasi-hydrostatically compressed in a diamond anvil cell; it also occurs to a much larger extent when powders are pressed uniaxially into discs and, notably, persists after pressure is released. We attribute the expansion to the insertion of H2O molecules and have identified shear-induced slipping of the nanosheets comprising multilayered MXene particles as a possible cause of this behavior in the latter case. This both has implications for the processing of MXenes and contributes to the field of materials with pseudo-negative compressibility by adding a new member for further investigation.

Two-dimensional Mo1.33C MXene with divacancy ordering prepared from parent 3D laminate with in-plane chemical ordering.

The exploration of two-dimensional solids is an active area of materials discovery. Research in this area has given us structures spanning graphene to dichalcogenides, and more recently 2D transition metal carbides (MXenes). One of the challenges now is to master ordering within the atomic sheets. Herein, we present a top-down, high-yield, facile route for the controlled introduction of ordered divacancies in MXenes. By designing a parent 3D atomic laminate, (Mo2/3Sc1/3)2AlC, with in-plane chemical ordering, and by selectively etching the Al and Sc atoms, we show evidence for 2D Mo1.33C sheets with ordered metal divacancies and high electrical conductivities. At ∼1,100 F cm-3, this 2D material exhibits a 65% higher volumetric capacitance than its counterpart, Mo2C, with no vacancies, and one of the highest volumetric capacitance values ever reported, to the best of our knowledge. This structural design on the atomic scale may alter and expand the concept of property-tailoring of 2D materials.

Two-Dimensional Titanium Carbide MXene As a Cathode Material for Hybrid Magnesium/Lithium-Ion Batteries.

As an alternative to pure lithium-ion, Li+, systems, a hybrid magnesium, Mg2+, and Li+ battery can potentially combine the high capacity, high voltage, and fast Li+ intercalation of Li-ion battery cathodes and the high capacity, low cost, and dendrite-free Mg metal anodes. Herein, we report on the use of two-dimensional titanium carbide, Ti3C2Tx (MXene), as a cathode in hybrid Mg2+/Li+ batteries, coupled with a Mg metal anode. Free-standing and flexible Ti3C2Tx/carbon nanotube composite "paper" delivered ∼100 mAh g-1 at 0.1 C and ∼50 mAh g-1 at 10 C. At 1 C the capacity was maintained for >500 cycles at 80 mAh g-1. The Mo2CTx MXene also demonstrated good performance as a cathode material in this hybrid battery. Considering the variety of available MXenes, this work opens the door for exploring a new large family of 2D materials with high electrical conductivity and large intercalation capacity as cathodes for hybrid Mg2+/Li+ batteries.

Synthesis and Characterization of an Alumina Forming Nanolaminated Boride: MoAlB.

The 'MAlB' phases are nanolaminated, ternary transition metal borides that consist of a transition metal boride sublattice interleaved by monolayers or bilayers of pure aluminum. However, their synthesis and properties remain largely unexplored. Herein, we synthesized dense, predominantly single-phase samples of one such compound, MoAlB, using a reactive hot pressing method. High-resolution scanning transmission electron microscopy confirmed the presence of two Al layers in between a Mo-B sublattice. Unique among the transition metal borides, MoAlB forms a dense, mostly amorphous, alumina scale when heated in air. Like other alumina formers, the oxidation kinetics follow a cubic time-dependence. At room temperature, its resistivity is low (0.36-0.49 µΩm) and - like a metal - drops linearly with decreasing temperatures. It is also a good thermal conductor (35 Wm(-1)K(-1) at 26 °C). In the 25-1300 °C temperature range, its thermal expansion coefficient is 9.5 × 10(-6 )K(-1). Preliminary results suggest the compound is stable to at least 1400 °C in inert atmospheres. Moderately low Vickers hardness values of 10.6 ± 0.3 GPa, compared to other transition metal borides, and ultimate compressive strengths up to 1940 ± 103 MPa were measured at room temperature. These results are encouraging and warrant further study of this compound for potential use at high temperatures.

Synthesis of two-dimensional titanium nitride Ti4N3 (MXene).

We report on the synthesis of the first two-dimensional transition metal nitride, Ti4N3-based MXene. In contrast to the previously reported MXene synthesis methods - in which selective etching of a MAX phase precursor occurred in aqueous acidic solutions - here a molten fluoride salt is used to etch Al from a Ti4AlN3 powder precursor at 550 °C under an argon atmosphere. We further delaminated the resulting MXene to produce few-layered nanosheets and monolayers of Ti4N3Tx, where T is a surface termination (F, O, or OH). Density functional theory calculations of bare, non-terminated Ti4N3 and terminated Ti4N3Tx were performed to determine the most energetically stable form of this MXene. Bare and functionalized Ti4N3 are predicted to be metallic. Bare Ti4N3 is expected to show magnetism, which is significantly reduced in the presence of functional groups.