Synthesis of Ti1-xWx Solid Solution MAX Phases and Derived MXenes for Sodium-Ion Battery Anodes

One of the distinguishing features of MAX phases and their MXene derivatives is their remarkable chemical diversity. This diversity, coupled with the two-dimensional nature of MXenes, positions them as outstanding candidates for a wide range of electrochemical applications. In this study, we report the synthesis of M site T1-xWx solid solution MAX phases, specifically (Ti1-xWx)2AlC and (Ti1-xWx)3AlC2. The 211-type phase exhibited a disordered solid solution, whereas the 312-type phase displayed a more ordered structure, resembling an o-MAX arrangement, with W atoms preferentially occupying the outer planes. This specific ordering in the 312-type MAX phase is attributed to the unique electronic structure and atomic radius of W, indicating that these characteristics are crucial for the preferential occupation of the outer planes. Moreover, corresponding solid-solution MXenes, Ti2.4W0.6C2Tz and Ti1.6W0.4CTz, were synthesized via selective etching of MAX powder precursors containing 20% W. These MXenes were evaluated as sodium-ion battery anodes, with Ti1.6W0.4CTz 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.