High‐Capacity Metastable Li <i> _x </i> CoO ₂ Cathode Enabled by Coherent Intergrowth of T ^# 2 and O2 Phases

O2-type Li_xCoO₂ (O2-LCO) has recently attracted significant attention as a promising cathode material with superior electrochemical performance compared to its conventional O3-type counterpart. Due to its metastable nature, O2-LCO is typically synthesized via ion exchange under relatively mild thermal conditions; however, the structural evolution and resultant phase composition during this process, which critically affect the material's performance, remain insufficiently understood. Here, we systematically elucidate the interplay between composition, structure, and electrochemical performance in metastable LiCoO₂ synthesized via molten-salt ion exchange. We show that the Na content in the precursor not only dictates the final Li content in the product but also thermodynamically governs the phase transition pathway. Comprehensive long-range and local structural characterizations reveal the composite nature of ion-exchanged LCO, comprising T^#2, O2, and O3 phases, with their relative fractions determined by the initial Na content. Electrochemical measurements, supported by theoretical calculations, indicate that the optimal phase composite maximizes both Li content and T^#2 fraction while suppressing O3 formation, thereby enhancing Li⁺ diffusion kinetics and structural compatibility. These insights provide a fundamental basis for phase engineering in metastable cathode materials and practical guidelines for designing high-performance layered oxide cathodes.