Sub-1000°C Molten Salt Synthesis of Solid-Solution (Ti,V,Fe)2AlC MAX Phase from Pre-Alloyed Ti-6Al-4V toward Etchant-Tunable (Ti,V,Fe)2C MXenes

Solid-solution MXenes have emerged as a transformative class of 2D materials, offering unprecedented control over electronic structure and redox activity through M-site compositional tuning. However, their broader development remains constrained by the extreme synthesis temperatures (>1400°C) and complex stoichiometric mixing protocols required for conventional solid-state MAX phase synthesis, which result in secondary phase formation, elemental inhomogeneity, and limited scalability. This study addresses these challenges by introducing a low-temperature, pre-alloyed precursor strategy, the first successful synthesis of a quaternary (Ti,V,Fe) 2 AlC solid-solution MAX phase via molten salt-assisted synthesis at a substantially reduced temperature of 950°C, employing commercial Ti-6Al-4V alloy as the precursor to guarantee atomic-scale transition metal distribution. The resulting MAX phase was subsequently converted into (Ti,V,Fe) 2 C MXene via in-situ hydrofluoric acid (HF) etching using three fluoride salts (LiF, NaF, and NH 4 F), demonstrating that etchant chemistry is a powerful parameter for engineering MXene morphology and electrochemical behavior. A key finding of this work is the direct morphology controllability imparted by fluoride salt selection. LiF-assisted etching produces open, loosely aligned lamellae with expanded interlayer spacing (d ≈ 9.55 Å) and the highest specific capacitance of 126.9 F/g at 5 mV/s; NaF-assisted etching yields compact, well-ordered onion-like aggregates (d ≈ 9.45 Å) with superior interfacial charge-transfer kinetics (R ct = 1.58 Ω); and NH 4 F-assisted etching results in aggressively degraded, defect-rich morphology with significantly reduced performance (65.8 F/g). These results establish an energy-efficient, industrially scalable pathway for the production of morphology-engineered, multi-elemental MXene electrodes and provide a robust mechanistic framework linking etchant hydration chemistry to structural architecture and electrochemical function.