Ionic Mobility in Energy Materials: Through the Lens of Quasielastic Neutron Scattering

Many energy-related materials rely on ionic migration for their function. Examples include the flow of ions in battery or fuel cell electrolytes or the coupled flow of ions and electrons in electrodes and membrane materials. As such, understanding, controlling, and improving ionic migration is a major focus of modern materials science. Significant and invaluable insight into the structure of materials and the collective motion of their atoms (phonons) is routinely obtained by elastic (diffraction) and inelastic (INS) neutron scattering methods. Here we focus on quasielastic neutron scattering (QENS) which can give unique atomic-level information on dynamics in the solid state. QENS can be used to measure the length- and time-scales of both local and long-range ionic motion and to give detailed insight into migration pathways. The length- and time-scales probed are comparable to computational techniques such as molecular dynamics, meaning that QENS can help test and validate theory. The information provided is also highly complementary to techniques such as tracer diffusion measurements, conductivity measurements, impedance studies, and solid-state NMR. We provide an introduction to the theory and experimental methods for QENS, presenting the concepts in a language accessible to materials chemists. We then review the insights given by QENS studies on energy materials that show oxide, sodium, and lithium ion migration in the solid state.