Synthesis of Ti_1-xW_x Solid Solution MAX Phases and Derived MXenes for Sodium-Ion Battery Anodes

A distinguishing feature of MAX phases and their MXene derivatives is their remarkable chemical diversity. This diversity, coupled with the 2D nature of MXenes, positions them as outstanding candidates for a wide range of electrochemical applications. Chemical disorder introduced by a solid solution can improve electrochemical behavior. Up to now, adding considerable amount of tungsten (W) in MAX phase and MXenes solid solutions, which can enhance electrochemical performance, proved challenging. In this study, the synthesis of M site Ti_1-xW_x solid solution MAX phases are reported. The 211-type (Ti_1-xW_x)₂AlC exhibits a disordered solid solution, whereas the 312-type (Ti_1-xW_x)₃AlC₂ displays a near-ordered structure, resembling o-MAX, with W atoms preferentially occupying the outer planes. Solid-solution MXenes, Ti_2.4W_0.6C₂T_z, and Ti_1.6W_0.4CT_z, are synthesized via selective etching of high-purity MAX powder precursors containing 20% W. These MXenes are evaluated as sodium-ion battery anodes, with Ti_1.6W_0.4CT_z showing exceptional capacity, outperforming existing multilayer MXene chemistries. This work not only demonstrates the successful integration of W in meaningful quantities into a double transition metal solid solution MAX phase, but also paves the way for the development of cost-effective MXenes containing W. Such advancements significantly widen their application spectrum by fine-tuning their physical, electronic, mechanical, electrochemical, and catalytic properties.