Simultaneous derivatization and exfoliation of a multilayered Ti 3 C 2 T x MXene into amorphous TiO 2 nanosheets for stable K-ion storage.

Two-dimensional transition metal compounds (2D TMCs) have been widely reported in the fields of energy storage and conversion, especially in metal-ion storage. However, most of them are crystalline and lack active sites, and this brings about sluggish ion storage kinetics. In addition, TMCs are generally nonconductors or semiconductors, impeding fast electron transfer at high rates. Herein, we propose a facile one-step route to synthesize amorphous 2D TiO 2 with a carbon coating (a-2D-TiO 2 @C) by simultaneous derivatization and exfoliation of a multilayered Ti 3 C 2 T x MXene. The amorphous structure endows 2D TiO 2 with abundant active sites for fast ion adsorption and diffusion, while the carbon coating can facilitate electron transport in an electrode. Owing to these intriguing structural and compositional synergies, a-2D-TiO 2 @C delivers good cycling stability with a long-term capacity retention of 86% after 2000 cycles at 1.0 A g -1 in K-ion storage. When paired with Prussian blue (KPB) cathodes, it exhibits a high full-cell capacity of 50.8 mA h g -1 at 100 mA g -1 after 140 cycles, which demonstrates its great potential in practical applications. This contribution exploits a new approach for the facile synthesis of a-2D-TMCs and their broad applications in energy storage and conversion.