Lithium-sulfur (Li-S) battery is one of the most promising energy storage systems because of its high specific capacity of 1675 mAh g(-1) based on sulfur. However, the rapid capacity degradation, mainly caused by polysulfide dissolution, remains a significant challenge prior to practical applications. This work demonstrates that a novel Ni-based metal organic framework (Ni-MOF), Ni6(BTB)4(BP)3 (BTB = benzene-1,3,5-tribenzoate and BP = 4,4'-bipyridyl), can remarkably immobilize polysulfides within the cathode structure through physical and chemical interactions at molecular level. The capacity retention achieves up to 89% after 100 cycles at 0.1 C. The excellent performance is attributed to the synergistic effects of the interwoven mesopores (∼2.8 nm) and micropores (∼1.4 nm) of Ni-MOF, which first provide an ideal matrix to confine polysulfides, and the strong interactions between Lewis acidic Ni(II) center and the polysulfide base, which significantly slow down the migration of soluble polysulfides out of the pores, leading to the excellent cycling performance of Ni-MOF/S composite.

Developing approaches to effectively induce and control the magnetic states is critical to the use of magnetic nanostructures in quantum information devices but is still challenging. Here we have demonstrated, by employing the density functional theory calculations, the existence of infinite magnetic sheets with structural integrity and magnetic homogeneity. Examination of a series of transition metal dichalcogenides shows that the biaxial tensile strained NbS(2) and NbSe(2) structures can be magnetized with a ferromagnetic character due to the competitive effects of through-bond interaction and through-space interaction. The estimated Curie temperatures (387 and 542 K under the 10% strain for NbS(2) and NbSe(2) structures, respectively) suggest that the unique ferromagnetic character can be achieved above room temperature. The self-exchange of population between 4d orbitals of the Nb atom that leads to exchange splitting is the mechanism behind the transition of the spin moment. The induced magnetic moments can be significantly enhanced by the tensile strain, even giving rise to a half-metallic character with a strong spin polarization around the Fermi level. Given the recent progress in achieving the desired strain on two-dimensional nanostructures, such as graphene and a BN layer, in a controlled way, we believe that our calculated results are suitable for experimental verification and implementation, opening a new path to explore the spintronics in pristine two-dimensional nanostructures.

Voltage and capacity fading of layer structured lithium and manganese rich (LMR) transition metal oxide is directly related to the structural and composition evolution of the material during the cycling of the battery. However, understanding such evolution at atomic level remains elusive. On the basis of atomic level structural imaging, elemental mapping of the pristine and cycled samples, and density functional theory calculations, it is found that accompanying the hoping of Li ions is the simultaneous migration of Ni ions toward the surface from the bulk lattice, leading to the gradual depletion of Ni in the bulk lattice and thickening of a Ni enriched surface reconstruction layer (SRL). Furthermore, Ni and Mn also exhibit concentration partitions within the thin layer of SRL in the cycled samples where Ni is almost depleted at the very surface of the SRL, indicating the preferential dissolution of Ni ions in the electrolyte. Accompanying the elemental composition evolution, significant structural evolution is also observed and identified as a sequential phase transition of C2/m → I41 → Spinel. For the first time, it is found that the surface facet terminated with pure cation/anion is more stable than that with a mixture of cation and anion. These findings firmly established how the elemental species in the lattice of LMR cathode transfer from the bulk lattice to surface layer and further into the electrolyte, clarifying the long-standing confusion and debate on the structure and chemistry of the surface layer and their correlation with the voltage fading and capacity decaying of LMR cathode. Therefore, this work provides critical insights for design of cathode materials with both high capacity and voltage stability during cycling.

The energetics and length scales associated with the interaction between point defects (vacancies and self-interstitial atoms) and grain boundaries in BCC Fe was explored. Molecular statics simulations were used to generate a grain boundary structure database that contained ≈ 170 grain boundaries with varying tilt and twist character. Then, vacancy and self-interstitial atom formation energies were calculated at all potential grain boundary sites within 15Å of the boundary. The present results provide detailed information about the interaction energies of vacancies and selfinterstitial atoms with symmetric tilt grain boundaries in iron and the length scales involved with absorption of these point defects by grain boundaries. Both low and high angle grain boundaries were effective sinks for point defects, with a few low-Σ grain boundaries (e.g., the Σ3{112} twin boundary) that have properties different from the rest. The formation energies depend on both the local atomic structure and the distance from the boundary center. Additionally, the effect of grain boundary energy, disorientation angle, and Σ-designation on the boundary sink strength was explored; the strongest correlation occurred between the grain boundary energy and the mean point defect formation energies. Based on point defect binding energies, interstitials have ≈ 80% more grain boundary sites per area and ≈ 300% greater site strength than vacancies. Last, the absorption length scale of point defects by grain boundaries is over a full lattice unit larger for interstitials than for vacancies (mean of 6-7Å vs. 10-11Å for vacancies and interstitials, respectively).