Author Archives: Andreas Röckert

Importance of the Ion-Pair Lifetime in Polymer Electrolytes

Authors: Harish Gudla, Yunqi Shao, Supho Phunnarungsi, Daniel Brandell, and Chao Zhang

imageIon pairing is commonly considered as a culprit for the reduced ionic conductivity in polymer electrolyte systems. However, this simple thermodynamic picture should not be taken literally, as ion pairing is a dynamical phenomenon. Here we construct model poly(ethylene oxide)–bis(trifluoromethane)sulfonimide lithium salt systems with different degrees of ion pairing by tuning the solvent polarity and examine the relation between the cation–anion distinct conductivity σ+–d and the lifetime of ion pairs τ+– using molecular dynamics simulations. It is found that there exist two distinct regimes where σ+–d scales with 1/τ+– and τ+–, respectively, and the latter is a signature of longer-lived ion pairs that contribute negatively to the total ionic conductivity. This suggests that ion pairs are kinetically different depending on the solvent polarity, which renders the ion-pair lifetime highly important when discussing its effect on ion transport in polymer electrolyte systems.

J. Phys. Chem. Lett. 2021, 12, 35, 8460–8464

Modelling Bulk Electrolytes and Electrolyte Interfaces with Atomistic Machine Learning

Authors: Yunqi Shao, Lisanne Knijff, Florian M. Dietrich, Kersti Hermansson,  Chao Zhang

imageBatteries and supercapacitors are electrochemical energy storage systems which involve multiple time-scales and length-scales. In terms of the electrolyte which serves as the ionic conductor, a molecular-level understanding of the corresponding transport phenomena, electrochemical (thermal) stability and interfacial properties is crucial for optimizing the device performance and achieving safety requirements. To this end, atomistic machine learning is a promising technology for bridging microscopic models and macroscopic phenomena. Here, we provide a timely snapshot of recent advances in this area. This includes technical considerations that are particularly relevant for modelling electrolytes as well as specific examples of both bulk electrolytes and associated interfaces. A perspective on methodological challenges and new applications is also discussed.

Batteries & Supercaps 2021, 4, 585.

Using DFTB to Model Photocatalytic Anatase–Rutile TiO2 Nanocrystalline Interfaces and Their Band Alignment

Authors: Verena Kristin Gupta, Bálint Aradi, Kyoung Kweon, Nathan Keilbart, Nir Goldman, Thomas Frauenheim, and Jolla Kullgren

Band alignments between Rutile and AnataseBand alignment effects of anatase and rutile nanocrystals in TiO2 powders lead to electron–hole separation, increasing the photocatalytic efficiency of these powders. While size effects and types of possible alignments have been extensively studied, the effect of interface geometries of bonded nanocrystal structures on the alignment is poorly understood. To allow conclusive studies of a vast variety of bonded systems in different orientations, we have developed a new density functional tight-binding parameter set to properly describe quantum confinement in nanocrystals. By applying this set, we found a quantitative influence of the interface structure on the band alignment.

J. Chem. Theory Comput. 2021, 17, 8, 5239–5247