Low-temperature magnetic properties and high-temperature diffusive behavior of LiNiO2 investigated by muon-spin spectroscopy

In order to elucidate the effect of Ni ions in the Li layer on magnetism and Li diffusion of LiNiO2, we have measured muon-spin rotation and muon-spin relaxation (mu+SR) spectra for the polycrystalline Li1-xNi1+ xO2 samples with x=0.02, 0.03, and 0.15. Weak transverse-field-mu+SR measurements demonstrated the existence of a bulk ferromagnetic transition at T-m=48(6) K for the x=0.03 sample and 161(7) K for x=0.15 while the x=0.02 sample exhibited an antiferromagnetic transition at 18(4) K. Zero-magnetic-field-(ZF) mu+SR measurements below T-m clarified the formation of static ferromagnetic (FM) order for the x=0.03 and 0.15 samples but only a highly disordered antiferromagnetic (AF) order for the x=0.02 sample. Therefore, the variation in the low-T magnetism with x is most unlikely due to the change in the concentration of an AF NiO-type domain or an FM Ni-rich cluster but likely due to a homogeneous change in the whole system. In the paramagnetic state, ZF- and longitudinal-field-mu+SR spectra exhibited a dynamic nuclear field relaxation. From the temperature dependence of the field fluctuation rate, a diffusion coefficient of Li+ ions (D-Li) at 300 K was estimated about 0.39(3) x 10(-11) cm(2)/s for the x=0.02 sample and 0.12(7) x 10(-11) cm(2)/s for x=0.15. On the other hand, the related compound, LiCrO2, did not show any diffusive behavior even at the highest temperature measured (=475 K). Considering the hindrance of diffusion by Ni in the Li+ diffusion plane and the fact that LiCrO2 is electrochemically inactive, the estimated D-Li is thought to be very reasonable for the positive electrode material of Li-ion batteries. Furthermore, at low temperatures where the Li+ ions are static, the internal magnetic field was still found to be fluctuating, due to a dynamic local Jahn-Teller distortion of the Ni3+ ions in a low-spin state with S=1/2(t(2g)(6)e(g)(1)).