Mxenes: A New Family of Two-Dimensional Materials and Its Application As Electrodes for Li and Na-Ion Batteries

            Two-dimensional, 2D, materials, such as graphene, possess a unique morphology that endows them with interesting and novel properties. Currently, the number of non-oxide materials that have been exfoliated is limited to two fairly small groups, viz. hexagonal, van der Waals bonded structures (e.g. graphene and BN) and layered transition metal chalcogenides.

            The MAX phases are a well established family of machinable layered ternary carbides and/or nitrides, with a composition of M_n+1AX_n, where M is an early transition metal, A is one of A group elements, X is C and/or N; with n = 1, 2, or 3. Selective etching of the A layers from the MAX phases results in exfoliating of the latter producing 2D layers of transition metals carbides and/or nitrides. We labeled the resulting 2D M_n+1X_n layers "MXenes" to emphasize the loss of the A group element from the MAX phases and the suffix "ene" to emphasize their 2D nature and their similarity to graphene.

            The etching process is typically carried out using aqueous hydrofluoric acid at room temperature. Nine different MXenes have been reported to date, viz., Ti₂C, Nb₂C, V₂C, (Ti_0.5,Nb_0.5)₂C, Ti₃C₂, (V_0.5,Cr_0.5)₃C₂, Ti₃CN, Ta₄C₃, and Nb₄C₃. The as-synthesized MXenes are terminated with a mixture of OH, O, and/or F groups.

            When select MXenes were tested as electrode materials for Li-ion batteries (LIBs), each phase showed unique electrochemical characteristics. In principle, some could be used as cathodes while others as anodes for LIBs. All the tested MXenes showed excellent capability to handle quite high cycling rates (as high as 36 C) as electrode materials for LIBs, which suggest they can be used in Li-ion capacitors as well. The lithiation and delithiation mechanisms were found to be due to redox intercalation/deintercalation reactions.

            As synthesized MXenes can be pressed, at room temperature, into freestanding discs, > 300 µm  thick. Some of these discs can be used as electrodes for LIBs with high areal capacities exceeding 5 mAh/cm². In addition to LIBs, some MXene also showed good capacity as electrode material for other ion batteries such as Na-ion batteries.

            Herein the progress in the synthesis of MXenes, in addition to their performance as electrode materials in LIBs and Na-ion batteries will be discussed, with special emphasis on the lithiation and delithiation mechanisms for Ti₃C₂.