Lithium-rich manganese-based layered oxides, such as Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ (LNCM), are promising cathodes that possess both ultrahigh-specific capacity and high working voltage, yet they have not been commercialized due to intrinsic challenges. These challenges include sluggish Li-ion diffusion kinetics and structural degradation from the layered structure to a spinel-like form, leading to poor power capability and severe capacity fading. To address these challenges, we have developed a modification technique that involves fusing LaF₃-decorated with La/F-doping. Protecting the electrode/electrolyte interface, inert LaF₃ particles decorate on the LNCM surface impede interfacial side reactions. La-doping near the surface enlarges the Li layer spacing and facilitates Li-ion diffusion. A stronger TM-F bond is formed when F diffuses into the bulk phase and replaces O, thereby decreasing the migration of TM. The resulting LaF₃-modified LNCM exhibits a high capacity of 256.1 mAh g⁻¹ at 0.1C and a high-power capability of 3555 W kg⁻¹ at 10C, which is 2.5 times higher than the pristine sample. We also elucidate the relationship between the structures (i.e., oxidation state of elements, bond energy, mechanical properties, and conductivity) and electrochemical behaviors (i.e., impedance and Li-ion diffusion) to explain why the modification of LaF₃ improves the power property of the LNCM.