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During the $\mu^-$SR measurements on Li-ion battery materials, a part of implanted $\mu^-$ is naturally captured by Li nucleus, leading to the formation of muonic-Li species in target materials. The past $\mu^-$SR study on Li metal shows the lack of relaxation in the TF-$\mu^-$SR spectrum at room temperature [1], despite the presence of large nuclear magnetic fields at the muonic-Li position from the surrounding Li nuclei. Such phenomenon was explained by diffusion of a He-like muonic-Li in Li metal. On the other hand, the He diffusion in solids is reported as a thermally activated process based on a thermochronometry gas analysis [2]. Therefore, the TF relaxation rate in Li is expected to increase with decreasing temperature, as the muonic-Li diffusion is suppressed at low temperatures.
Nevertheless, the $\mu^-$SR experiment on both natural Li and $^6$Li indicates the absence of detectable TF relaxation rates even at the lowest temperature measured. This clearly excludes the scenario that the muonic-Li start to diffuse at a certain temperature below 300 K. Although the mechanism on such a zero TF relaxation rate is not fully understood, we could ignore the contribution from the muonic-Li on the asymmetry $\mu^-$SR spectrum for the Li-ion battery materials.
[1] D. Favart et al., Phys. Rev. Lett. ${\bf 25}$, 1348 (1970).
[2] C. Huber et al., Geochimica et Cosmochimica Acta ${\bf 75}$, 2170 (2011).
