With the increasing interest in highly concentrated electrolyte systems, correct determination of the cation transference number is important. Pulsed‐field gradient NMR technique, which measures self‐diffusion coefficients, is often applied on liquid electrolytes because of the wide accessibility and simple sample preparation. However, since the assumptions of this technique, that is, complete salt dissociation, all ions participating in motion, and all of them moving independently, no longer hold true in concentrated solutions, the transference numbers, thus obtained are often over‐estimated. In the present work, impedance spectroscopy at a frequency range of 1 MHz to 0.1 mHz was used to examine the concentration effect on lithium‐ion transference number under anion‐blocking conditions for two electrolytes: lithium bis(fluorosulfonyl)imide (LiFSI) in sulfolane (SL) and lithium bis(trifluorosulfonyl)imide (LiTFSI) in tetraglyme (G4). The of the former was almost an order of magnitude higher than that of the latter. It also appeared to increase with increasing concentration while the latter followed an opposite trend. The faster Li⁺ transport in the SL system is attributed to the formation of a liquid structure consisting of extended chains/bridges of SL molecules and the anions, which facilitate a cation‐hopping/ligand‐exchanged‐typed diffusion mechanism by partially decoupling the cations from the anions and solvent molecules. The G4 system, in contrast, is dominated by the formation of long‐lived, stable [Li(G4)]⁺ solvation cages that results in a sluggish Li⁺ transport. The difference between the two transport mechanisms is discussed via comparison of the bulk ionic conductivity, viscosity, ion self‐diffusion coefficients, and the Onsager transport coefficients.

Solid Electrolyte Interphase (SEI) has been identified as the most important and least understood component in lithium-ion batteries. Despite extensive studies in the past two decades, a few mysteries remain: what is the chemical form of and degree of mobility of Li⁺ in the interphase? What fraction of Li⁺ is permanently immobilized in the SEI, while the rest are still able to participate in the cell reactions via the ion-exchange process with Li⁺ in the electrolyte? This study attempted to answer, in part, these questions by using ⁶Li and ⁷Li-isotopes to label SEIs and electrolytes, and then quantifying the distribution of permanently immobilized and ion-exchangeable Li⁺ with solid-state NMR and ToF-SIMS. The results showed that the majority of Li⁺ were exchanged after one SEI formation cycle, and a complete exchange after 25 cycles. Ion exchange by diffusion based on concentration gradient in the absence of applied potential also occurred simultaneously. This knowledge will provide a foundation for not only understanding but also designing better SEIs for future battery chemistries.

Nearly five decades of effort has focused on identifying and developing new polymer capacitor films for higher-than-ambient temperature applications, but simultaneous demands of processability, dielectric permittivity, thermal conductivity, dielectric breakdown strength, and self-clearing capability limit the number of available materials. Demands on these criteria are even more stringent in growing numbers of applications demanding high power performance. Aromatic polyimides, though not a panacea, are a class of heat-resistant polymers of great interest to researchers as capacitor dielectrics because of good thermal and mechanical stability. In this chapter, the key aspects and advantages of metallized polymer film capacitors are compared to analogous alternative technologies (polymer-film-metal-foil, ceramic, and electrolytic capacitors), followed by a comprehensive review of commercial resin development leading up to recent research on polyimides targeted for operating temperature above 150°C. Finally, this chapter provides a brief discussion on the recent effort on combining computation and synthesis to design polymers with desirable dielectric properties.