Molten salt-shielded solid-state synthesis (MS5) reaction-driven >99% pure Ti3AlC2 MAX phase: effect of MAX phase purity on the interlayer separation of MXenes and Na-ion storage

Abstract

Ti₃C₂-X MXenes have attracted tremendous research interest because their 2D laminar morphology provides numerous functional applications. The application options rely on the purity and interlayer spacing of MXenes, which eventually depend on the purity of the MAX phase. This motivated us to synthesize pure MAX phases to produce MXenes at large scale using simpler and less expensive techniques. However, producing prerequisite pure MAX phases at atmospheric pressure and relatively lower temperatures is still a challenging task. This study reports the synthesis of thermally stable pure MAX phases (and MXenes) by systematically varying the molar ratio of the precursors, reaction temperature, and reaction time using a novel MS⁵ process under atmospheric pressure conditions. The purity of MAX phases under different synthesis conditions shows a direct interrelationship with the 2D laminar morphology of MXenes. The highest purity of >99% for the MAX phase was achieved by reacting Ti : Al : C precursors in a 3 : 1.5 : 1.9 molar ratio at 1350 °C for 90 min and delivered highly distinctive 2D nanostructured laminar MXenes with larger interlayer spacing and specific surface area (i.e., 8.8 ± 0.1 m² g⁻¹), which are superior to those in previously reported works. These results reveal that the precursor molar ratios and reaction conditions tailor the MAX phase purity, thereupon, they are crucial for achieving good quality 2D laminar MXenes with large specific surface areas, further enhancing the performance of MXene-related devices. The developed MS⁵-MXenes, when probed as electrodes for Na ion applications, show excellent initial and reversible capacity values of 142 mA h g⁻¹ and 103 mA h g⁻¹ (at 50 mA g⁻¹), respectively.