Nb2C and Nb2CO2 MXenes as Anodes in Li-Ion Batteries: A Comparative Study by First-Principles Calculations.

The new generation of Li-ion batteries is based on integrating 2D materials into the electrodes to increase the energy density while reducing the charging time and size. The two-dimensional transition metal carbide or nitride (MXene) materials offer ideal electronic properties, such as metallic behavior, low energy barriers for Li-ion diffusion, and structural stability. This study focuses on Nb2C and Nb2CO2 MXenes, which have shown promising Li-storage capacity, especially the oxidized phase. By using density functional theory (DFT) and thermodynamic criteria, we studied the Li intercalation process in both MXenes. The results show that the Li intercalation process in the oxidized phase is more stable. Also, the Li diffusion barriers are 35 and 250 meV for the bare and oxidized phase, due to the strong interaction between Li ions and O functional groups. Nb2C and Nb2CO2 MXenes deliver a maximum gravimetric theoretical capacity of 275 and 233.26 mA h/g, respectively, with a stable performance.

Atomic-Scale Description of the Magnetic TiN|FeCo Multilayers.

Motivated by the experimental findings of Wolff et al., we investigated the TiN|FeCo multilayers at the atomic scale. Four different models were employed to investigate the interface, considering both Fe and Co surface terminations of the FeCo compounds. The interface formation energy formalism was employed to study the thermodynamic stability of these models. The results show that an interface mediated by Co atoms is most likely to appear in the experiment. Also, the Fe surface termination is more viable than a Co surface termination. The magnetic moments of Co at the interface are 1.48 µB/atom, which denotes a decay compared to bulk (1.76 µB/atom). Besides, Ti acquires a very small induced magnetization of -0.05 µB/atom. Our proposed atomistic model of the TiN|FeCo multilayer system fits perfectly with the structure obtained in experiments, and it is a step forward in the theoretical-experimental design of wear-resistant coatings with outstanding magnetic and mechanical properties.

Strain Effects on the Two-Dimensional Cr2N MXene: An Ab Initio Study.

Structural, electronic, and magnetic properties of two-dimensional Cr2N MXene under strain were studied. The uniaxial and biaxial strain was considered from -5 to 5%. Phonon dispersion was calculated; imaginary frequency was not found for both kinds of strain. Phonon density of states displays an interesting relation between strain and optical phonon gaps (OPGs), that it implies tunable thermal conductivity. When we apply biaxial tensile strain, the OPG increases; however, this is not appreciable under uniaxial strain. The electronic properties of the dynamically stable systems were investigated by calculating the band structure and electron localization function (ELF) along the (110) plane. The band structure showed a metallic behavior under compressive strain; nevertheless, under tensile strain, the system has a little indirect band gap of 0.16 eV. By analyzing, the ELF interactions between Cr-N are determined to be a weaker covalent bonding Cr2N under tensile strain. On the other hand, if the Cr atoms reduce or increase their self-distance, the magnetization alignment changes, also the magnetic anisotropy energy displays out-of-plane spin alignment. These properties extend the potential applications of Cr2N in the spintronic area as long as they can be grown on substrates with high lattice mismatch, conserving their magnetic properties.