Abstract: Lithium-rich layered oxides (LLOs) are prospective materials for future-generation cathodes attributable to their high specific capacity. However, significant surface instability, particularly under high-voltage operating conditions, leads to substantial voltage decay and dramatic capacity degradation during long-term cycling, severely limiting their widespread application. In this study, we developed a universal brine quenching strategy to construct a stabilized composite surface structure for LLOs. This structure comprises an inner surface layer with a Y-doped layered structure and an outermost layer featuring a disordered rock-salt structure. Doping in the layered structure strengthens the Y–O bonds, raises the energy barrier for oxygen evolution, and significantly increases the stability of the lattice oxygen. Additionally, the disordered rock-salt surface structure reduces oxygen release during the charge and discharge cycles. Consequently, this well-designed surface structure significantly boosts the structural stability of the LLO surface, suppresses structural degradation during long-term cycling, and facilitates Li⁺ diffusion kinetics. The improved redox activity, combined with superior structural stability, contributes to an outstanding electrochemical performance. For instance, the Y-quenched Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode exhibited an improved discharge capacity of 283 mAh·g⁻¹ at 0.1 C and 223 mAh·g⁻¹ at 1 C, along with remarkable cyclic stability retaining 91.2% of its capacity after 300 cycles at 1 C, and a reduced voltage decay of 0.76 mV per cycle (compared to 1.16 mV per cycle for pristine LLO). This research provides valuable insights into the design and synthesis of high-energy-density LLOs through a simple and cost-effective strategy.