Elena Degoli

Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lithium-ion batteries (LiBs), mainly due to their higher electrochemical stability and wider electrochemical window. Since they are not naturally electroactive, in order to allow their use in LiBs, it is necessary to mix the ionic liquids with lithium salts (Li). Li–PF6, Li–BF4, and Li–TFSI are among the lithium salts more frequently used in LiBs, and each anion, namely PF6 (hexafluorophosphate), BF4 (tetrafluoroborate), and TFSI (bis(trifluoromethanesulfonyl)azanide), has its own solvation characteristics and interaction profile with the pyrrolidinium ions. The size of Pyr cations, the anion size and symmetry, and cation–anion combinations influence the Li-ion solvation properties. In this work, we used molecular dynamics calculations to achieve a comprehensive view of the role of each cation–anion combination and of different fractions of lithium in the solutions to assess their relative advantage for Li-ion battery applications, by comparing the solvation and structural properties of the systems. This is the most-comprehensive work so far to consider pyrrolidinium-based ILs with different anions and different amounts of Li: from it, we can systematically determine the role of each constituent and its concentration on the structural and dynamic properties of the electrolyte solutions.

The interfacial structure of a silicon grain boundary (Si-GB) plays a decisive role on its chemical functionalization and has implications in diverse physical–chemical properties of the material. Therefore, the GB interface is particularly relevant when the material is employed in high performance technological applications. Here, we studied from first principles the role of GB interface by providing an atomistic understanding of two different Σ3{112} Si-GB models. These models are (1×1) and (1×2) Σ3{112} Si-GBs, which lead to different structural reconstruction. Starting from these two models, we have shown that geometry optimization has an important role on the structural reconstruction of the GB interface and, therefore, on its properties. For this reason, we discussed different methodologies to define an optimal relaxation protocol. The influence of the local structures in (1×1) and (1×2) models has also been investigated in the presence of vacancies where different light impurities of different valency (C, N, H, O) can segregate. We studied how local structures in (1×1) and (1×2) models are modified by the presence of vacancies and impurities. These structural modifications have been correlated with the changes of the energetics and electronic properties of the GBs. The behavior of (1×1) and (1×2) models was demonstrated to be significantly different. The interaction with vacancies and the segregation of C, N, H, and O are significantly different depending on the type of local structures present in Σ3{112} Si-GB.