Enhanced Electrochemical Behavior Through Transformation of Solid-Solution MXenes into Complex Oxides

In this work, we will present the MXene-to-oxide transformation mechanism using (V_2-<i>y</i>Nb_y)CT<i>_x</i> solid-solution MXenes. We will describe how the structure can be controlled through tuning of synthetic parameters and highlight the unique physical and electrochemical properties to the oxide derivatives in non-aqueous Li-ion batteries. Using two-dimensional MXenes for transition metal oxide synthesis is attractive due to their diverse compositions and nanoscale morphology, which can be leveraged to produce complex nanostructures. Incorporating multiple transition metals in solid-solution MXenes can lead to complex oxide derivatives with intergrown components that form tight heterointerfaces, improving their energy storage properties.<br/>The formation of MXene-derived oxides produced in this work involved a three-step process, starting from the synthesis of the single-metal and solid-solution (Nb<i>_y</i>V_2-<i>y</i>)AlC MAX phases (y = 0.00, 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75, 2.00). The (Nb<i>_y</i>V_2-<i>y</i>)CT<i>_x</i> MXenes were then obtained using a mixed-acid HF/HCl etchant. As the Nb/V ratio approached 1, MAX phase X-ray diffraction (XRD) features in the MXene XRD patterns gradually diminished and disappeared, indicating improved purity of the MXene compositions similar to Nb_1.00V_1.00CT<i>_x</i>. This better conversion from MAX to MXene for these compositions (y = 0.75, 1.00, 1.25) may be driven by the formation of defects in the MAX phase crystal lattice caused by lattice mismatch of V and Nb atoms in the M layers, which can allow for better etchant access to remove the A layer. The (Nb<i>_y</i>V_2-<i>y</i>)CT<i>_x</i> MXenes were then transformed into oxides using hydrogen-peroxide-initiated dissolution followed by hydrothermal treatment process. XRD patterns of the V₂CT_x- and vanadium-rich (y = 0.25, 0.50) (Nb<i>_y</i>V_2-<i>y</i>)CT<i>_x</i>-derived oxides show the formation of bilayered vanadium oxide (V₂O₅●<i>n</i>H₂O, BVO), while the patterns of Nb₂CT_x- and niobium-rich (y = 1.75, 1.50) (Nb<i>_y</i>V_2-<i>y</i>)CT<i>_x</i>-derived oxides showed nanostructured amorphous niobium oxide (nANO) formation. For solid-solution (Nb<i>_y</i>V_2-<i>y</i>)CT<i>_x</i> MXene derivatives with a Nb/V ratio close to 1 (y = 0.75, 1.00, 1.25), a mixture of BVO and nANO features indicated the formation of composite structures. Scanning electron microscopy of the Nb_1.00V_1.00CT<i>_x</i>-derived oxide revealed nANO particles embedded in the BVO matrix, forming tight heterointerfaces between the two materials. Electrochemical cycling of the (Nb<i>_y</i>V_2-<i>y</i>)CT<i>_x</i>-derived oxides in non-aqueous Li-ion cells was conducted in a 1.0 – 4.0 V vs. Li/Li⁺ potential window to encompass the electrochemically active regions for BVO and nANO. The V₂CT<i>_x</i>-derived oxide showed redox processes typical for BVO electrodes, while the Nb₂CT<i>_x</i>-derived oxide exhibited typical pseudocapacitive behavior for niobium oxides. The solid-solution MXene-derived oxides primarily showed composite-like behavior with some notable exceptions. The vanadium-rich Nb_0.25V_1.75CT<i>_x</i>-derived oxide exhibited a discharge capacity of 297 mAh g^-1, along with new redox behavior distinct from BVO and nANO, likely due to substitutional doping of Nb in the BVO structure. The Nb_1.00V_1.00CT<i>_x</i>-derived oxide achieved a discharge capacity of 298 mAh g^-1, with an additional pseudocapacitive response attributed to the heterointerfacial growth between the BVO and nANO particles. Heterointerfacial effects were also observed during extended cycling, where the V_1.00Nb_1.00CT<i>_x</i>-derived oxide exhibited three times the capacity retention of a 1:1 physical mixture of V₂CT<i>_x</i>-derived and Nb₂CT<i>_x</i>-derived oxides. These results demonstrate that transforming solid-solution MXene into oxide derivatives enables the design of unique two-dimensional layered materials that leverage doping and heterointerfacial growth. These derivatives can lead to novel functionalities in energy storage, which cannot be achieved through traditional composite fabrication.