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Hydrogen enhanced decohesion is expected to play a major role in ferritic steels, especially at grain boundaries. Here, we address the effects of some common alloying elements C, V, Cr, and Mn on the H segregation behaviour and the decohesion mechanism at a Σ 5 ( 310 ) [ 001 ] 36.9 ∘ grain boundary in bcc Fe using spin polarized density functional theory calculations. We find that V, Cr, and Mn enhance grain boundary cohesion. Furthermore, all elements have an influence on the segregation energies of the interstitial elements as well as on these elements’ impact on grain boundary cohesion. V slightly promotes segregation of the cohesion enhancing element C. However, none of the elements increase the cohesion enhancing effect of C and reduce the detrimental effect of H on interfacial cohesion at the same time. At an interface which is co-segregated with C, H, and a substitutional element, C and H show only weak interaction, and the highest work of separation is obtained when the substitute is Mn.

The main advantage of Mg batteries over other metal counterparts is its ability to work with a pure metallic anode, achieving a very high specific capacity. Unfortunately, pure Mg is hard to machine due to its brittleness, making it extremely difficult to produce foils that are thin enough for practical battery applications. Alloying Mg with small amounts of doping elements can enhance its ductility. However, care should be given to ensure that the dopants do not interfere with the electrochemical process of plating and stripping of Mg from the anode during battery operation. Dopants should prefer to be in bulk or at a stacking fault rather than migrating to the surface to meet this requirement. In this work, we carried out a computational screening of 34 dopants that are reported to reduce Mg brittleness to check which of them energetically prefers to stay in bulk. We found that only 12 out of the 34 meet such a criterion. Y and Nd, two of the main dopants in the WE43 commercial alloys, are among the 12 beneficial doping elements, which presents a practical avenue for the exploration for superior Mg battery anode material.

Rechargeable Mg–S batteries are attractive for next-generation energy storage devices due to their high theoretical energy density (1684 W h kg–1 and 3286 W h L–1) and low costs. The poor cycling performance of Mg–S batteries is linked to the formation of the solid discharge products, i.e., MgS2 and MgS, which are electronic and ionic insulators. However, the formation of MgS itself contradicts such a premise because it requires further oxidation of MgS2. Indeed, the insulating nature of MgS2 should inhibit such an oxidation process in the first place. Using first-principles calculations and ab initio molecular dynamics simulations, we evaluate the charge transport associated with point defects in MgS2 and MgS. In MgS2, the single-electron polaron is the most abundant type of defect that emerges from our model, which appears at a low concentration at thermodynamic equilibrium but displays high mobility. However, under conditions far from thermodynamic equilibrium, mimicking those for battery operations, the concentration of electron polarons increases, enhancing the electronic conductivity in MgS2. We demonstrate that in regimes far from thermodynamic equilibrium, the single-electron polarons coalesce to form double-electron polarons, whose mobilities are similar to that of a single-electron polaron. MgS2 holds electronic conduction through a polaron migration mechanism for ≤3 μm thick deposits, enabling further oxidation to form MgS. For MgS, our model suggests that the doubly positive Mg interstitial and doubly negative Mg vacancy are identified as the prevalent defects with high concentrations. However, due to the low mobility of these defects, their contribution to charge transport is negligible, which stops the oxidation process and severely hinders battery cyclability. Our results indicate that rechargeable Mg–S batteries can be developed if we ensure that the battery discharge does not push the oxidation process beyond the formation of MgS2.