In-situ synthesis of niobium-doped TiO₂ nanosheet arrays on double transition metal MXene (TiNbCT_x) as stable anode material for lithium-ion batteries

Two-dimensional layered MXene material with high conductivity and good mechanical flexibility has gained wide attention in the field of energy storage. However, the practical application of MXene was hampered by its limited specific capacity and the unstable structure. Herein, a composite of in-situ grown niobium-doped TiO₂ nanosheet arrays on a double transition metal MXene TiNbCT_x (TiNbC@NTO) was successfully obtained via a hydrothermal pretreatment followed by in-situ partial oxidation strategy. The prepared TiNbC@NTO combines multiple advantages of both MXene and oxide, including high conductivity derived from the unoxidized MXene TiNbCT_x, superior structure stability from the in-situ produced oxide between the MXene layers, which prevents structural collapse and restacking during charging and discharging, and the large layer space which promotes lithium-ion transport. The degree of oxidation of MXene can be adjusted by controlling the reaction temperature, and the oxide nonosheet turn dense with the increase of the temperature. All the oxidized MXenes show improved electrochemical performance compared with the pure TiNbCT_x, and the TiNbC@NTO-500 with the appropriate degree of oxidation exhibits the highest reversible capacity, best cycling stability of 261 mAh g^-1 after 500 cycles at 1.0 A g^-1 among all the as-prepared composites. Furthermore, an extraordinary rate performance (148.5 mAh g^-1 at 2 A g^-1) was obtained based on the pseudocapacitance dominated mechanism. This work provides a new insight into improving the performance of MXene-based anode material for lithium-ion batteries.