Negative muons used alongside positive muons to measure lithium diffusion in battery materials
29 Jul 2020
- Rosie de Laune



For the first time, a combination of positive- and negative-muon spin rotation and relaxation (μ±SR) has been used to measure lithium-ion diffusion in a magnetic battery material.

​Left: The crystal structure of LiMnPO4 in orthorhombic symmetry. Large yellow spheres show Li, medium blue spheres in octahedra show Mn, and medium purple spheres in tetrahedra show P, and small red and magenta spheres represent the two μ+ sites,predicted by density functional theory (DFT) calculations with VASP. The corners of each polyhedron are occupied by O (not shown).
Right: The relationship between ν± (DLi) and inverse temperature. ν± (DLi) estimated from the μ±SR results. The solid line represents the fit using a thermal activation process.​​
Credit: Jun Sugiyama

Using positive muon spin relaxation and rotation (μ+SR) to investigate battery materials is a well-established technique at ISIS, and other facilities, due to its ability to measure the diffusion of charge carriers such as Li+, as described in our muon energy materials feature article. However, at higher temperatures, the μ+ that is implanted in the material can itself diffuse through the material. This leads to ambiguity when distinguishing between Li+ and μ+ mobility in positive muon diffusion measurements above such temperatures.

When using negative muons, the implanted μ- species won't diffuse, even at high temperatures, enabling the study of other charge carriers in the lattice. This study uses the combination of negative muon experiments on ARGUS to determine the species that is diffusing, and positive muon experiments on EMU to study the detailed temperature dependence of this diffusion.

The international research collaboration chose the battery cathode material LiMnPO4 as the focus of this study, as previous μ+SR results show diffusion that begins to increase with temperature above 200 K. By using the combined μ±SR techniques, the researchers were able to confirm that is lithium ions that are diffusing in the material, and that the implanted μ+ species remain stable.

The ability of μ±SR to provide such a unique insight opens up the possibility for its use for measuring other charge transfer materials, even those containing magnetic ions. These materials range from other lithium ion battery materials, as well as those containing sodium or potassium ions, and beyond batteries to other energy applications including materials for fuel cells and solar cells.

Lead researcher Jun Sugiyama explains; "Since our discovery that μ+SR detects ion diffusion in solids in 2009, we have considered the effect of possible μ+ diffusion on the obtained results. Finally, we have clarified μ+SR ambiguity with μ-SR! ISIS is a suitable facility for such purpose, because both μ+SR and μ-SR, i.e., μ±SR are available in one building."​

Further information

The full paper can be found online at DOI: 10.1103/PhysRevResearch.2.033161​​

A feature article about the use of positive muons in energy materials is on our website

Contact: de Laune, Rosie (STFC,RAL,ISIS)