In order to elucidate the effect of Ni ions in the Li layer on magnetism and Li diffusion of ${\text{LiNiO}}_2$, we have measured muon-spin rotation and muon-spin relaxation $\left({\mu }^{+}\text{SR}\right)$ spectra for the polycrystalline ${\text{Li}}_{1-x}{\text{Ni}}_{1+x}{\text{O}}_2$ samples with $x=0.02$, 0.03, and 0.15. Weak transverse-field-${\mu }^{+}\text{SR}$ measurements demonstrated the existence of a bulk ferromagnetic transition at $T_{\text{m}}=48\left(6\right)\text{K}$ 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) ${\mu }^{+}\text{SR}$ measurements below $T_{\text{m}}$ 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-${\mu }^{+}\text{SR}$ spectra exhibited a dynamic nuclear field relaxation. From the temperature dependence of the field fluctuation rate, a diffusion coefficient of ${\text{Li}}^{+}$ ions $\left(D_{\text{Li}}\right)$ at 300 K was estimated about $0.39\left(3\right)\times {10}^{-11}{\text{cm}}^2/\text{s}$ for the $x=0.02$ sample and $0.12\left(7\right)\times {10}^{-11}{\text{cm}}^2/\text{s}$ for $x=0.15$. On the other hand, the related compound, ${\text{LiCrO}}_2$, did not show any diffusive behavior even at the highest temperature measured $\left(=475\text{K}\right)$. Considering the hindrance of diffusion by Ni in the ${\text{Li}}^{+}$ diffusion plane and the fact that ${\text{LiCrO}}_2$ is electrochemically inactive, the estimated $D_{\text{Li}}$ is thought to be very reasonable for the positive electrode material of Li-ion batteries. Furthermore, at low temperatures where the ${\text{Li}}^{+}$ ions are static, the internal magnetic field was still found to be fluctuating, due to a dynamic local Jahn-Teller distortion of the ${\text{Ni}}^{3+}$ ions in a low-spin state with $S=1/2\left(t_{2g}^6{e}_g^1\right)$.

• Received 26 September 2010

DOI:https://doi.org/10.1103/PhysRevB.82.224412