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The demand for high-performance and cost-effective lithium-ion batteries (LIBs) calls for cathodes with high energy density, structural stability, and reduced reliance on costly Co and Ni. Here, a Co-free and Ni-minimized (≤10 mol%) Li- and Mn-rich layered oxide cathode is presented, Li1.2Mg0.1Ni0.1Mn0.6O2, engineered to balance performance and cost. Low-cost Mg2+ substitution can stabilizes the lattice and mitigates voltage decay. However, together with Ni minimization, it suppresses the initial oxygen redox, lowering first-cycle capacity and energy density. Importantly, high-voltage activation during the initial cycle successfully triggers the latent oxygen redox, and remarkably, enables full capacity recovery in subsequent cycles. This pre-activation not only restores performance but also mitigates voltage decay and structural degradation over prolonged cycling. The Li1.2Mg0.1Ni0.1Mn0.6O2 delivers a discharge capacity of ≈276.6 mAh g−1 and an energy density of ≈902.2 Wh kg−1, with ≈93.4% capacity retention after 100 cycles. Operando X-ray diffraction reveals a minimal c-axis variation (≈0.13%) and provides evidence of suppressed structural disorder following pre-activation. Supported by electrochemical measurements, structural analysis, and first-principles calculations, these findings unlock a pathway toward cost-effective, high-energy layered cathodes with stable cycling performance for next-generation LIBs.