Towards the energy materials of the future: a walk through ionic conduction with neutrons

Ionic conduction is one of the most important processes in energy materials. Understanding it in detail is key for developing more performant rechargeable batteries and fuel cells. A recently published article provides a comprehensive combined tutorial and research review of Quasielastic Neutron scattering (QENS) experiments on oxide, lithium and sodium ion conductors. Much of the work has been done at the ILL.

New energy materials are essential in the global challenge to address climate change and major environmental issues, and the quest for better performing materials and devices is a very active field of research. In particular, solid electrolytes are an essential component in devices such as rechargeable batteries and fuel cells.

Electrolytes conduct electricity due to the presence of mobile ions. For example, normal salt (NaCl) dissociates into Na+ and Cl- ions when dissolved in water, and the solution is a conductor. Solid electrolytes are crystals in which certain ions are mobile, or polymers. While having a lower conductivity at room temperature, solid electrolytes have considerable advantages in what concerns safety, stability and durability. Ionic conduction, the mobility of charged ions through solid electrolytes, is thus a central process in energy materials. A deep understanding of ionic conduction is key in energy material research, and neutrons can be an ideal probe.

When neutrons scatter from a crystalline material, the observed energy spectrum typically shows an elastic scattering line (no energy exchanged with sample) and a set of energy lines due to inelastic scattering (corresponding to energy exchanges of characteristic values). Quasi elastic neutron scattering (QENS) is a limiting case of inelastic neutron scattering: there is a very small energy transfer between the incident neutrons and the atoms in the sample, which is observed as a broadening of the elastic energy line. Such tiny energy transfers can be caused by dynamic effects in the sample - namely diffusive dynamics, where the thermal movement of atoms results in their net transport. This is exactly the situation in ionic conduction.

QENS is thus an ideal experimental technique to probe ionic conduction, with a range of instruments providing information across picosecond and nanosecond time-scales, as well as key geometric information about the characteristic length-scales of ionic mobility e.g. the jump distances within and between metal-oxide polyhedra with variable coordination environments.

The Endurance Programme

The ILL Endurance upgrade programme was completed in 2024 with a budget of about 50 ME delivering more than 30 upgrades and new instruments over eight years. For QENS, the three time-of-flight spectrometers – IN5, SHARPER and PANTHER – now define the state-of-the-art, while for backscattering – IN16B – a novel, variable focusing guide has been developed to greatly enhance the performance of the instrument in time-of-flight (BATS) mode. In addition, new perspectives are provided by the wide-angle spin echo instrument (WASP) that opens up this particular QENS technique with much shorter measurement times and therefore higher throughput.

Image: Length- and time-scales accessible to neutrons, compared to other experimental methods.

“QENS has a proven track record but also considerable potential in this field, offering insight that is complementary to widely-used, lab-based techniques such as conductivity measurements and solid state NMR,” explains Ivana Radosavljevic Evans, from Durham University, the main author of the publication, adding “For this reason, the article is intended to be pedagogical to attract new users to the technique which is only available at large-scale facilities, like the ILL.” The aim was to write in a way that was both rigorous and accessible to chemists, physicists and materials scientists who are not experts on QENS or neutron scattering in general.”

Most of the experiments discussed in the publication have been done at the ILL. Indeed, such measurement require low energy (or ‘cold’) neutrons, and over the last decades, the ILL has developed a comprehensive set of cold neutron instruments (see box for further details).

“It is the continuous upgrade of instruments delivering ever better performance – typically a factor of ten improvement each decade – that has allowed the QENS technique to move from its historical focus on the dynamics of hydrogen and protons, which are very strong neutron scatterers, to much weaker scattering elements.”, says Mark Johnson, one of the authors of the publication.

A key feature in recent QENS studies is the systematic use of computer simulation of the molecular dynamics that provide models of dynamics that are consistent with the experimental data. In the case of oxide ion conductors, specific simulations have been used to properly account for bonding fluctuations that must take place when ions migrate through metal oxide crystal structures. “A former ILL Science Director coined the expression ‘more than simply neutrons’ to emphasise how infrastructure and support is required to produce cutting-edge scientific results,” recalls Mark Johnson.

Perspectives for future work are highly encouraging, with investments in instrumentation at facilities around the world, such as the Endurance programme at the ILL, and new sources, like the European Spallation Source (ESS). Increased performance will allow smaller samples to be studied sooner after their discovery and facilitate high throughput campaigns for screening new materials.

Reference: Bettina Schwaighofer, Miguel A. Gonzalez, Mark R. Johnson, John S. O. Evans, Ivana Radosavljević Evans. 'Ionic Mobility in Energy Materials: Through the Lens of Quasielastic Neutron Scattering', Chemistry of Materials, Vol. 37, Issue 10 (2025).

pubs.acs.org/doi/full/10.1021/acs.chemmater.5c00238

ILL contacts: Mark Jonhson, Bettina Schwaighofer, Miguel A. Gonzalez

Computing for Science at the ILL

The Computing for Science (CS) group at ILL - through its ‘Computation Lab (C-lab)’ initiative - provides comprehensive support for a wide range of simulation methods that cover many of the systems and materials studied with neutrons at ILL. The support involves simulation expertise, a computational cluster, simulation software and, crucially, software that allows simulated ‘experimental data’ to be derived from simulations and therefore directly compared with real experimental data. CS/C-lab also organises training sessions and workshops. For more information contact (rebolini@remove-this.ill.fr).