As Li ion battery (LIB) technology develops, its nanoscale design is attracting more and more interest. Atomic layer deposition (ALD) has emerged as a promising technique to provide conformal and ultrathin coating for LIB materials. In this work, an iron oxide (FeOx) ALD was performed on LiMn1·5Ni0·5O4 (LMNO) cathode particles, followed by an annealing process. Fe was doped into LMNO during the ALD process; and it was found that lattice oxygen loss and structural change of LMNO occurred at a reaction temperature of 450 °C under vacuum. Meanwhile, Fe diffused from the surface to the bulk of LMNO. After post-annealing, FeOx-coated LMNO recovered its structure and lattice oxygen under proper annealing conditions, and exhibited significant enhancement of specific capacity, rate capability, and cyclic stability.