(Invited) Lithium Ions in Layered 2D Transition Metal Carbides: MAX Phases and Mxenes
Authors: Gogotsi Yury , Gao Yu , Xu Jianguang 
Published: February 2020

Recently, a family of layered ternary transition-metal carbides and/or nitrides, known as the MAX phases, received increasing attention as precursors for two-dimensional (2D) transition-metal carbides and/or nitrides known as MXenes. The MAX phases have the general formula Mn+1AXn, where n= 1 to 3 and M is an early transition metal, A is an A-group element, and X is carbon and/or nitrogen.1
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2 By selectively etching the A-element layers from MAX phases, MXenes have been synthesized. Given that the MXene surfaces are terminated with OH, and F groups resulting from the etching process, it is more accurate to refer to them as Mn+1XnTx, where T represents the surface terminating groups, such as O, F and OH. There are more than 70 compounds known in the MAX family to date (plus dozens of solid solutions),3 most of which offer good electronic conductivity, lamellar structure, and temperature and environmental stability. As-prepared MXenes exist in a multilayered structure, which appears as stacked multilayer flakes. The multilayered structures can be delaminated into single-layer flakes by intercalation and/or sonication.4
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5 Due to their high specific surface areas, metallic conductivity, and hydrophilic surfaces, MXenes have shown promising performance as electrodes for supercapacitors. MXenes are also promising anode materials for LIBs due to their excellent conductivity and cation intercalation capability.6
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7 Nb4C3 MXene shows stable capacity, also at high rates (Figure 1). A capacity of 410 mAh g−1 for a Ti3C2 MXene “paper” anode at 1C rate without adding any binder has been reported. This Ti3C2 MXene “paper” was fabricated by intercalating Ti3C2 with dimethyl sulfoxide, bath sonication for 6 h, and filtering the Ti3C2 colloidal solution.8
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9 The experimental procedure is very simple. MXene-based LIB anodes exhibited Li-ion capacity up to 800 mAh g−1, good rate performance, and excellent cycling stability.9
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10 These results suggest a potential for MXene-supported hybrid electrodes for energy storage applications.
By decreasing the particle size, we successfully demonstrated that not only MXenes but also their precursors - MAX phases, such as Ti2SC and Ti3SiC2, exhibit Li-ion storage capacity and show promise as LIB anode materials. Capacities of 180 and 150 mAh g−1 were achieved by submicrometer size Ti2SC and Ti3SiC2, respectively, accompanied by good cycle life and excellent rate performance.11 It opens the door to exploring a large family of potential anode materials.
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Figure 1