Unveiling the Tunable Li Diffusion in MXenes Bilayers: Insights from First-Principles Study

The properties of 2D transition-metal carbides (MXenes) with the composition M2CT2 (where M = Sc, Ti, V, Nb, and Cr; and T = O and F) have been investigated. Employing first-principles calculations based on density functional theory, our study comprehensively explores the attributes of monolayer and bilayer MXenes. We focus on characteristics such as stability, diffusion, and selectivity of a lithium atom, along with cointercalation phenomena within Ti2CO2 bilayers. In the context of monolayers, we examine six distinct adsorption sites for functional atoms (T) on the surfaces of M2CT2. Energetically favorable monolayer configurations are identified for subsequent investigations involving bilayers. Structural modifications, including variations in layer types and stacking shifts, potentially alter the properties of the M2CT2 multilayer. The role of bilayer interfaces is crucial in understanding the insertion effects of Li atoms between layers as interlayer spacing can be dynamically adjusted through intercalation processes. We explore the structural relaxation of Ti2CO2 bilayers both with and without cointercalation. We analyze key lithium atom diffusion properties, such as diffusion barriers, diffusion rates, and relative diffusion rates (selectivity). Our findings reveal the tunability of the diffusion barrier in relation to interlayer spacing. By introducing specific cointercalated metals, such as potassium, the Li diffusion barrier can be dramatically reduced from 0.36 eV to approximately 0.01 eV. This reduction occurs as the interlayer spacing increases from 3.4 to 4.1 Å, enabling a Li atom to move nearly free between the Ti2CO2 layers. Notably, larger interlayer spacing gradually elevates the diffusion barrier toward the monolayer value as the Li atom prefers surface migration on either layer. Our work presents a comprehensive computational framework for understanding cointercalation in MXenes bilayers, shedding light on the intricate interplay between the structure and Li mobility.