Layered Orthorhombic Nb₂O₅@Nb₄C₃T<i>_x</i> and TiO₂@Ti₃C₂T<i>_x</i> Hierarchical Composites for High Performance Li‐ion Batteries

Engineering electrode nanostructures is critical in developing high‐capacity, fast rate‐response, and safe Li‐ion batteries. This study demonstrates the synthesis of orthorhombic Nb 2 O 5 @Nb 4 C 3 T x (or @Nb 2 CT x ) hierarchical composites via a one‐step oxidation —in flowing CO 2 at 850 °C —of 2D Nb 4 C 3 T x (or Nb 2 CT x ) MXene. The composites possess a layered architecture with orthorhombic Nb 2 O 5 nanoparticles decorated uniformly on the surface of the MXene flakes and interconnected by disordered carbon. The composites have a capacity of 208 mAh g −1 at a rate of 50 mA g −1 (0.25 C) in 1–3 V versus Li + /Li, and retain 94% of the specific capacity with 100% Coulombic efficiency after 400 cycles. The good electrochemical performances could be attributed to three synergistic effects: (1) the high conductivity of the interior, unoxidized Nb 4 C 3 T x layers, (2) the fast rate response and high capacity of the external Nb 2 O 5 nanoparticles, and (3) the electron “bridge” effects of the disordered carbon. This oxidation method was successfully extended to Ti 3 C 2 T x and Nb 2 CT x MXenes to prepare corresponding composites with similar hierarchical structures. Since this is an early report on producing this structure, there is much room to push the boundaries further and achieve better electrochemical performance.