Spin polarized beam for battery materials research: μ±SR and β-NMR

Spin polarized particles exhibiting beta decay, such as, muon and Li-8 are very useful for detecting internal magnetic fields in solids. Here, I review our attempts to use these spin polarized beams for battery materials research. Due to a unique lifetime and gyromagnetic ratio of muon, a positive muon spin rotation and relaxation (mu+SR) technique is very popular for studying microscopic internal magnetic fields in condensed matters caused by electron and nuclear magnetism. Particularly in 2009, it was found that Li+ ion diffusion in LixCoO2, which is the most common cathode material in a Li-ion battery, is detectable with mu+SR even under the presence of paramagnetic Co ions (Sugiyama et al., Phys. Rev. Lett. 103, 147601, 2009), while NMR is unable to do so due to the effect of localized magnetic moments on a spin-lattice relaxation rate. Such finding opened the door for mu+SR on energy materials research. Since then, many battery materials have been investigated with mu+SR in order to determine their intrinsic diffusion coefficient (D) of Li+ and Na+ ions. Furthermore, using such intrinsic D, the other important parameters are successfully derived, such as, the reactive surface area, diffusion pathway, and density of mobile ions. In 2018, we have initiated to measure internal magnetic fields in solids with a negative muon, i.e. a mu-SR technique, in J-PARC (Sugiyama et al., Phys. Rev. Lett. 121, 087202, 2018). This will provide a new insight for microscopic internal magnetic fields from lattice sites, while mu+SR does so from interstitial sites. On the contrary, a long lifetime of Li-8 (1.21 s) makes a Li-8 beta-NMR technique suitable for detecting both concentration and diffusion of Li in solids. Particularly in TRIUMF, since the implantation energy of Li-8 is tunable from almost 0 to about 28 keV, we can measure both spin-lattice relaxation rate and spin-spin relaxation rate as a function of depth up to about 200 nm, as well as mu+SR using a low energy muon beam in PSI. These will clarify whether the space charge layer of Li+ exists at the interface between electrode and electrolyte material.