By Eric Sigmund Northwestern University Evanston, Illinois
The ability to push a button and instantly perform otherwise tedious data analysis tasks has streamlined the study of electrolytic materials used in batteries. One of the most crucial parts in the study and development of polymer and amorphous ionic electrolytes is a detailed understanding of the free ions' motion within the substance. Nuclear magnetic resonance (NMR) provides an excellent tool to address this issue because it can selectively measure the behavior of a single atomic species among a veritable zoo of ions and molecules. Northwestern University researchers have developed a series of macro programs that provide significant time savings by quickly calculating spin lattice relaxation times (T1) that are crucial to evaluating new electrolytic materials.
How do the polymer and various synthetic molecules of the amorphous ionic electrolytes impede or make way for the ions' motion? How can we choose additive molecules to optimize that motion? How high of a conductivity can we attain via these engineered molecular dynamics? Northwestern University researchers are addressing these issues - which are critical to the development of improved batteries - by applying NMR to study the ionic motion.
Specificity of NMR
The specificity of NMR is of great use as we try to follow the motion in space and time of one of the primary charge carriers in our electrolyte: lithium cations. NMR is a probe of the magnetic field distribution occurring within a material and once a nuclear species is chosen, the evolution of that species can be studied spectroscopically. The chosen nuclear species has an intrinsic magnetic moment similar to a small bar magnet. This intrinsic moment evolves by "precessing" like a top around whatever local field that is experienced by the species. A spectrum of frequencies, and thus magnetic fields, is acquired. This gives clues as to the structure of the material, to the nuclear motion within the structure, and to various interactions within.
In our case, we seek to measure the ionic motion. The NMR tools take advantage of the evolution of the whole group of lithium cations as they move around the material sampling magnetic. In particular, we have performed what are called "relaxation measurements", as well as some preliminary "diffusion measurements" that provide a direct measure of the motion itself. The relaxation measurements, which mostly consist of one class called "spin-lattice relaxation", measure the interactions of the spins with each other or the external environment, in terms of time scales called relaxation times.