AbstractDeveloping sacrificial cathode prelithiation technology to compensate for active lithium loss is vital for improving the energy density of lithium‐ion battery full‐cells. Li₂CO₃ owns high theoretical specific capacity, superior air stability, but poor conductivity as an insulator, acting as a promising but challenging prelithiation agent candidate. Herein, extracting a trace amount of Co from LiCoO₂ (LCO), a lattice engineering is developed through substituting Li sites with Co and inducing Li defects to obtain a composite structure consisting of (Li_0.906Co_0.043▫_0.051)₂CO_2.934 and ball milled LiCoO₂ (Co‐Li₂CO₃@LCO). Notably, both the bandgap and Li─O bond strength have essentially declined in this structure. Benefiting from the synergistic effect of Li defects and bulk phase catalytic regulation of Co, the potential of Li₂CO₃ deep decomposition significantly decreases from typical >4.7 to ≈4.25 V versus Li/Li⁺, presenting >600 mAh g⁻¹ compensation capacity. Impressively, coupling 5 wt% Co‐Li₂CO₃@LCO within NCM‐811 cathode, 235 Wh kg⁻¹ pouch‐type full‐cell is achieved, performing 88% capacity retention after 1000 cycles.