Abstract
A versatile group of 2D carbide materials from the past decade, MXenes, have attracted attention for their rich chemistry and wide range of properties. The perhaps best known MXene, namely, Ti3C2Tx, has been observed to stack in two distinct ways, and simulations show that this influences interlayer bonding energy and Li diffusion. In this DFT study, six types of Ti3C2T2 interlayer bonds resulting from O, F, and OH termination groups are assessed with respect to stability. It is shown that OH termination groups are highly stable up to 50% coverage, but unstable for higher coverage. A model to predict stacking type based on termination group chemistry shows that the degree of hydrogen bonding is the deciding factor. The model is also tested on V2CT2 and Zr3C2T2, giving similar results to those of Ti3C2T2. By calculating migration barriers for Ti3C2O2, we show that Li, Na, and Mg have orders of magnitude faster diffusion in the stacking favored by hydrogen bonds. XRD patterns calculated for both stackings show they are close to indistinguishable, highlighting the need for caution when classifying stacking.
Highlights
A versatile group of 2D carbide materials from the past decade, MXenes, have attracted attention for their rich chemistry and wide range of properties
Looking at the structure of MAX phase Ti3AlC2,9 it can be seen that the trigonal stacking corresponds to the stacking obtained if the Ti3C2 layers in MAX phase Ti3AlC2 do not move during synthesis of Ti3C2Tx
While Thygesen et al.[16] primarily investigated how well different van der Waals functionals described the Ti2CO2 MXene, they showed that stacking influences the migration barrier of Li in Ti2CO2 and found that a trigonal stacking gave orders of magnitude faster migration than an octahedral stacking
Summary
Ti3C2O2 and that there are compositions where the stackings are close in energy so that a mixture of both stackings may be expected. This work has showed that intercalation kinetics for Li/Na/Mg, termination group chemistry, and stacking are all strongly dependent on one another. This will inspire investigations combining electrochemical cycling, termination group measurements, and STEM imaging to cast further light on MXene properties and how stacking and termination groups can be tuned to control these properties. All calculations were performed with the plane wave DFT code Vienna Ab Initio Simulation package (VASP),[26−28] version 5.4.4, with the functional being GGAtype PBEsol[29] described by the projector augmented wave method (PAW).[30] The cutoff for the plane waves was 450 eV
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