authors contributed equally to this work. One Sentence Summary: Lewis acidic molten salts etching is an effective and promising route for producing MXenes with superior electrochemical performance in non-aqueous electrolyte. Abstract: Two-dimensional carbides and nitrides of transition metals, known as MXenes, are a fast-growing family of 2D materials that draw attention as energy storage materials. So far, MXenes are mainly prepared from Al-containing MAX phases (where A = Al) by Al dissolution in F-containing solution, but most other MAX phases have not been explored. Here, a redox-controlled A-site-etching of MAX phases

During the past decade, two-dimensional (2D) materials have attracted extensive attention due to their high surface area to volume ratio, unique electronic structures and physiochemical properties derived from their low dimensionality. [1][2][3][4][5][6][7] Graphene, the most studied 2D material with ultrahigh mechanical strength, [8][9] excellent electronic and thermal conductivities, [1,[10][11] exhibits potential applications in electrochemical energy storage, transparent electrodes, and nano-composites. [12] However, due to its intrinsic zero bandgap and simple chemistry, applications of graphene are restricted in some aspects, such as field effect transistors.[10] Thus, investigations on other 2D materials are performed, especially for those 2D materials with two or more composition elements, such as metal oxide, layered metal chalcogenides (LMDCs), hexagonal boron nitride(BN), hydroxides, etc. [6,13] In recent years, a new class of 2D materials called MXenes, have emerged [14] Members in this family are described by the general formula of Mn+1XnTz (wherein M is an early transition metal, X is C and/or N, n is 1, 2, 3, and Tz denotes surface terminated functional groups). [14] Generally, MXenes are produced by the selective etching of Al layers from their parental layered ternary MAX phases, a large group which comprises more than 70 members. [15][16] Through selective etching of aluminium layers, experimental investigations have successfully identified about 10 different MXenes, Ti3C2Tz, Ti2CTz, Ta4C3Tz, TiNbCTz, (V0.5,Cr0.5)3C2Tz, Ti3CNTz, Nb2CTz, V2CTz, and Nb4C3Tz. [14,17] Most of the synthesized MXenes are metallic, [18] hydrophilic, and predicted to have high elastic moduli, implying potential application as reinforcement of polymer. [19][20] The existance of Dirac electrons in some MXenes has also been theoretically predicted .[21] Moreover, similar to graphene, MXenes are promising candidate electrode materials for lithium-ion batteries and supercapacitors by facile intercalation of Li ions into the MXene layers. [22][23][24][25][26][27] Recently, considerable efforts have been made to further expand the family of 2D carbides. On the basis of a substitutional solid solution method, Gogotsi and Barsoum et al. [28] successfully synthesized Mo2TiC2Tz, Mo2Ti2C3Tz, and Cr2TiC2TzMXenes, and surface dependent electrochemical behaviors in the case of Mo2TiC2Tz have been revealed. Moreover, Mo2CTz MXene [29] has been synthesized through selective etching of gallium (Ga) from a thin film of the new ternary nanolaminated Mo2Ga2C. [30][31] Besides, large-area high-quality 2D α-Mo2C, WC, and TaC crystals have been fabricated by a chemical vapour deposition (CVD) process.[32] However, potential MXene compounds in materials systems where Al-containing MAX phases are not established, such as Hf2C and Zr2C, are yet to be produced.Herein, for the first time, we report the preparation of Zr-containing 2D carbide based on selective extraction of Al-C units from an alternative layered ternary Zr3Al3C5, benefiting from ...

We demonstrate fabrication of a two-dimensional Hf-containing MXene, HfCT, by selective etching of a layered parent Hf[Al(Si)]C compound. A substitutional solution of Si on Al sites effectively weakened the interfacial adhesion between Hf-C and Al(Si)-C sublayers within the unit cell of the parent compound, facilitating the subsequent selective etching. The underlying mechanism of the Si-alloying-facilitated etching process is thoroughly studied by first-principles density functional calculations. The result showed that more valence electrons of Si than Al weaken the adhesive energy of the etching interface. The MXenes were determined to be flexible and conductive. Moreover, this 2D Hf-containing MXene material showed reversible volumetric capacities of 1567 and 504 mAh cm for lithium and sodium ions batteries, respectively, at a current density of 200 mAg after 200 cycles. Thus, HfCT MXenes with a 2D structure are candidate anode materials for metal-ion intercalation, especially for applications where size matters.