Negative Muon Spin Rotation and Relaxation Study on Battery Anode Material, Spinel Li₄Ti₅O₁₂

Abstract

To study an intrinsic self-diffusion coefficient of Li+ (DLiJ) in a battery anode material, spinel Li4Ti5O12, negative and positive muon spin rotation and relaxation (μ±SR) spectra were measured using a powder sample in a temperature range of 100–400 K. The implanted μ+ in this study locates at an interstitial site near the O2– ion to form an O–H like bond, whereas the implanted μ– is mainly captured by an oxygen nucleus, resulting in muonic oxygen formation. The μ+ at an interstitial site is known to diffuse at high temperatures. Nevertheless, the muonic oxygen is immobile until the spinel decomposition temperature. The field fluctuation rate determined with μ–SR (ν–) increased with increasing temperature above 200 K with a 0.08(5) eV thermal activation energy. The ν–(T) curve agreed well with the ν+(T) curve obtained with μ+SR. Thus, the origin of a fluctuation in the internal nuclear magnetic field was unambiguously confirmed as Li+ diffusion, i.e., the possibility of μ+ diffusion is clearly excluded. From the obtained ν±, DLiJ was estimated to be 8(2) × 10–12 cm2/s at 300 K with Ea = 0.08 eV, under the assumption that the Li+ ions at the regular tetrahedral site jump to the vacant octahedral site with a random walk process.