Enhancing the lithium storage properties of molten salt-etched Ti₃C₂T_x through sequential intercalation of alkali ions

Lewis acid molten salt etching of MAX phases has emerged as a universal route to synthesize fluorine-free MXenes. However, the layer-stacked structure and halogen-rich termination of such MXenes limit their widespread use in energy storage. Herein, a sequential ion intercalation strategy is proposed to modulate the interlayer structure of the molten salt-etched Ti3C2Tx MXene (MS-MXene) for improving its lithium storage performance. The sequential ion intercalation process involved immersing MS-MXene in mixed alkaline solutions of LiOH, NaOH, and KOH, allowing for the pre-intercalation of smaller Li+ ions, which then facilitated the subsequent intercalation of larger Na+ and K+ ions (Li/Na/K-MXene). Consequently, the interlayer spacing of MS-MXene experienced an expansion from 11.02 & Aring; to 11.22 & Aring;, endowing the Li/Na/K-MXene with abundant surface active sites and improved ion/electron transport capabilities. When configured as an anode for LIBs, the Li/Na/K-MXene exhibited a high capacity of 323.1 mA h g(-1) at 50 mA g(-1), a superior rate capability of 170.0 mA h g(-1) at 2000 mA g(-1), and robust cycling stability with no decay over 1200 cycles. The proposed approach shows promise for expanding to additional classes of MXenes and can potentially advance MS-MXene for practical energy storage applications.