Two-Dimensional Fe8N Nanosheets: Ferromagnets and Nitrogen Diffusion

Two-dimensional functionalized MBene Mg2B3T (T = O, H, and F) monolayers as anode materials for high-performance K-ion batteries

Two-dimensional metal borides have received attention as high performance battery anode materials. During the practical application, the 2D surface terminalization is an inevitable problem. This study employs first-principles calculations to investigate the termination of the Mg2B3 monolayer with O, H, F, and Cl groups. These structures' stabilities are examined through energetic, mechanical, kinetic and thermodynamic stability studies. Electronic property analysis shows that Mg2B3T (T = O, H, F, and Cl) monolayers are all metallic. Calculated results reveal that the Mg2B3O, Mg2B3H, and Mg2B3F monolayers exhibit high K ion storage capacities (up to 826 mA h g-1, 980 mA h g-1, and 804 mA h g-1, respectively), with diffusion barriers of 0.338 eV, 0.490 eV, and 0.507 eV, respectively. More importantly, the calculated in-plane lattice constants of the substrate materials exhibit a minimal variation and the observed volume expansion is almost negligible (less than 0.08%) during the entire potassization process, which is much lower than that of the pristine Mg2B3 monolayer. This structural stability is attributed to the presence of surface functional groups. These results provide helpful insights into designing and discovering other high-capacity anode materials for batteries.

Interlayer electronic coupling regulates the performance of FeN MXenes and Fe2B2 MBenes as high-performance Li- and Al-ion batteries

When two-dimensional (2D) materials are stacked into van der Waals structures, interlayer electronic coupling can induce excellent properties in energy storage materials. Here, we investigate the interlayer coupling of the FeN/Fe2B2 heterojunction as an anode material, which is constructed using vertically planar FeN and puckered Fe2B2 nanosheets. These structures were searched by the CALYPSO method and computed by density functional theory calculations. The stabilities of the FeN monolayer, Fe2B2 monolayer, and FeN/Fe2B2 heterojunction were tested in terms of dynamics, mechanics, and thermodynamics, respectively. These structures have good performances as anode materials, including the capacities of the FeN (Fe2B2) monolayer of 9207 mA h g-1 (2713 mA h g-1) and 3069 mA h g-1 (1005 mA h g-1) for Al and Li, respectively. The stable FeN/Fe2B2 heterojunction shows extremely low diffusion barriers of 0.01 eV, a high Al ion capacity of 4254 mA h g-1, and relatively low voltages. Hess's law revealed that the interlayer electronic coupling impacts the adsorption process of the FeN layer in the FeN/Fe2B2 heterojunction, which decreases the pz orbital of the N atom for the heterojunction. The unequal distribution of electrons between the layers results in interlayer polarization; the value of interlayer polarization was quantitatively calculated to be 0.64 pC m-1. The presence of adsorbed Li and Al atoms between the layers helps maintain the original structure and prevents the interlayer sliding from damaging the heterojunction. These findings offer insights for understanding the structural and electronic properties of the FeN/Fe2B2 heterojunction, which provides crucial information for rational design and advanced synthesis of novel electrode materials.

An antiferromagnetic semiconducting FeCN2 monolayer with a large magnetic anisotropy and strong magnetic coupling

Two-dimensional antiferromagnetic (AFM) materials with an intrinsic semiconductivity, a high critical temperature, and a sizable magnetic anisotropy energy (MAE) have attracted extensive attention because they show promise for high-performance spintronic nanodevices. Here, we have identified a new FeCN2 monolayer with a unique zigzag Fe chain through first-principles swarm structural search calculations. It is an AFM semiconductor with a direct band gap of 2.04 eV, a Néel temperature (TN) of 176 K, and a large in-plane MAE of 0.50 meV per Fe atom. More interestingly, the intrinsic antiferromagnetism, contributed by the strong magnetic coupling of neighbouring Fe ions, can be maintained under the external biaxial strains. A large cohesive energy and high dynamical stability favor synthesis and application. Therefore, these fascinating properties of the FeCN2 monolayer make it a promising nanoscale spintronic material.

High-pressure phases of a Mn-N system

Highly compressed extended states of light elemental solids have emerged recently as a novel group of energetic materials. The application of these materials is seriously limited by the energy-safety contradiction, because the material with high energy density is highly metastable and can hardly be recovered under ambient conditions. Recently, it has been found that high-energy density transition metal polynitrides could be synthesized at ∼100 GPa and recovered at ∼20 GPa. Inspired by these findings, we have studied a high-pressure Mn-N system from the aspects of structure, stability, phase transition, energy density and electronic structure theoretically for the first time. The results reveal that MnN₄_P1̄ consisting of [N₄]_∞^2- is thermodynamically stable at 36.9-100 GPa, dynamically stable at 0 GPa and has a noticeably high volumetric energy density of 15.71 kJ cm^-3. Upon decompression, this structure will transform to MnN₄_C2/m with the transition barrier declining sharply at 5-10 GPa due to the switching of transition pathways. Hence, we propose MnN₄_P1̄ as a potential energetic material that is synthesizable above 40 GPa and recoverable until 10 GPa.