The electronic structures and lithium diffusion in the cathode material Immm-Li₂FeO₂ of lithium-ion batteries are calculated by the first-principles method based on the density functional theory. The calculated results show that Immm-Li₂FeO₂ is ferromagnetic, and the band structure indicates a semi-metal character. The d-electrons of Fe ions are in the low spin state, with a spin polarization of 8.01%. The spin-up and spin-down band structure are also analyzed by using the l-decomposed electronic density of states. Furthermore, the energy barriers for the lithium ion diffusion in different directions are calculated by the nudged elastic band method. For comparison, the potential barriers for the Li₂MO₂ (M = Co, Ni, Cu) are also calculated. The results suggest that it is easier for Li ion to diffuse in the c-axis directionof Li₂FeO₂, with an energy barrier of only 0.1 eV. The energy barrier is 0.21 eV for Li to diffuse in the ab-axis direction, while the diffusion barrier is 0.39 eV along the a-axis direction of Li₂FeO₂. All these values of energy barriers are lower than those in other Fe-based cathodes mentioned, indicating that the Li diffusion coefficient in Immm-Li₂FeO₂ should be larger than those of other materials, which also indicates that the Li₂FeO₂ is of great importance as cathode material.