The development of high-voltage cathodes is very important to meet the pursuit of high energy and power density for next-generation lithium-ion batteries. LiMn1.5Ni0.5O4 (LMNO), as one of the most promising high-voltage cathodes, is still challenged by unstable surface chemistry, metal dissolution, and structure transition during charge/discharge cycling. In this work, a porous structure combined with Fe doping in LMNO was achieved by iron oxide (FeOx) atomic layer deposition (ALD) and postannealing. The formation of the porous structure was closely related to FeOx ALD, and the sites of Fe doping in LMNO spinel were also controlled by annealing temperature. After this modification, Li+ transport and cycling stability were both significantly improved.