Structure properties correlation in MXenes: 2D anodic materials for sodium ion batteries

Recently, MXenes [1], a class of 2D materials obtained by the chemical exfoliation of carbide MAX phases, have been proposed as suitable anodic materials for SIBs devices [2]. The Mn+1AXn, or MAX phases are transition-metal carbides and nitrides with a layered, hexagonal structure where ‘‘M’’ is a transition metal, ‘‘A’’ is a 13 or 14 group’s element, and ‘‘X’’ is C and/or N. M-X based MXenes can be obtained by the corresponding MAX phase by chemical or electrochemical etching of the A element, forming a layered structure with large space between two M-X sandwiches (around 1 nm) suitable for ion pseudo-intercalation. The lamellar structure of MXenes facilitates the intercalation of many alkaline and earth-alkali metal ions, on an extended range of charge-recharge rates for thousands number of cycles [3]. Although there are several papers on the use of MXenes as active materials in both SIBs and supercapacitors devices, the correlations among their structure, chemical composition, and the electrochemical properties are seldom investigated. In the present contribute we discuss how different etching processes of starting Ti3AlC2 phase to produce Ti3C2Tx (T=F, Cl, OH) have a deep influence on the chemical composition (Tx), the crystalline structure, and the morphologies, which in turns rule the electrochemical behavior towards the reaction with the sodium ion. The Ti3C2Tx phase obtained by chemical etching shows a reversible capacity of about 150 mAh g-1 with very low potential hysteresis and an average anodic potential of 1.4 V vs. Na+/Na (figure 1).